What s the difference between passing by reference vs passing by value

Question

What is the difference between    a parameter passed by reference a parameter passed by value    Could you give me some examples  please

User · Answer

Pass by value sends a COPY of the data stored in the variable you specify  pass by reference sends a direct link to the variable itself  So if you pass a variable by reference and then change the variable inside the block you passed it into  the original variable will be changed  If you simply pass by value  the original variable will not be able to be changed by the block you passed it into but you will get a copy of whatever it contained at the time of the call

User · Answer

Many answers here  and in particular the most highly upvoted answer  are factually incorrect  since they misunderstand what  call by reference  really means  Here s my attempt to set matters straight   TL DR  In simplest terms    call by value means that you pass values as function arguments call by reference means that you pass variables as function arguments   In metaphoric terms    Call by value is where I write down something on a piece of paper and hand it to you  Maybe it s a URL  maybe it s a complete copy of War and Peace  No matter what it is  it s on a piece of paper which I ve given to you  and so now it is effectively your piece of paper  You are now free to scribble on that piece of paper  or use that piece of paper to find something somewhere else and fiddle with it  whatever  Call by reference is when I give you my notebook which has something written down in it  You may scribble in my notebook  maybe I want you to  maybe I don t   and afterwards I keep my notebook  with whatever scribbles you ve put there  Also  if what either you or I wrote there is information about how to find something somewhere else  either you or I can go there and fiddle with that information    What  call by value  and  call by reference  don t mean  Note that both of these concepts are completely independent and orthogonal from the concept of reference types  which in Java is all types that are subtypes of Object  and in C  all class types   or the concept of pointer types like in C  which are semantically equivalent to Java s  reference types   simply with different syntax    The notion of reference type corresponds to a URL  it is both itself a piece of information  and it is a reference  a pointer  if you will  to other information  You can have many copies of a URL in different places  and they don t change what website they all link to  if the website is updated then every URL copy will still lead to the updated information  Conversely  changing the URL in any one place won t affect any other written copy of the URL   Note that C   has a notion of  references   e g  int amp   that is not like Java and C  s  reference types   but is like  call by reference   Java and C  s  reference types   and all types in Python  are like what C and C   call  pointer types   e g  int       OK  here s the longer and more formal explanation   Terminology  To start with  I want to highlight some important bits of terminology  to help clarify my answer and to ensure we re all referring to the same ideas when we are using words   In practice  I believe the vast majority of confusion about topics such as these stems from using words in ways that to not fully communicate the meaning that was intended    To start  here s an example in some C-like language of a function declaration   void foo int param        line 1   param    1      And here s an example of calling this function   void bar       int arg   1      line 2   foo arg          line 3     Using this example  I want to define some important bits of terminology    foo is a function declared on line 1  Java insists on making all functions methods  but the concept is the same without loss of generality  C and C   make a distinction between declaration and definition which I won t go into here  param is a formal parameter to foo  also declared on line 1 arg is a variable  specifically a local variable of the function bar  declared and initialized on line 2 arg is also an argument to a specific invocation of foo on line 3   There are two very important sets of concepts to distinguish here  The first is value versus variable    A value is the result of evaluating an expression in the language  For example  in the bar function above  after the line int arg   1   the expression arg has the value 1  A variable is a container for values  A variable can be mutable  this is the default in most C-like languages   read-only  e g  declared using Java s final or C  s readonly  or deeply immutable  e g  using C   s const     The other important pair of concepts to distinguish is parameter versus argument    A parameter  also called a formal parameter  is a variable which must be supplied by the caller when calling a function  An argument is a value that is supplied by the caller of a function to satisfy a specific formal parameter of that function   Call by value  In call by value  the function s formal parameters are variables that are newly created for the function invocation  and which are initialized with the values of their arguments   This works exactly the same way that any other kinds of variables are initialized with values  For example   int arg   1  int another variable   arg    Here arg and another variable are completely independent variables -- their values can change independently of each other  However  at the point where another variable is declared  it is initialized to hold the same value that arg holds -- which is 1   Since they are independent variables  changes to another variable do not affect arg   int arg   1  int another variable   arg  another variable   2   assert arg    1     true assert another variable    2     true   This is exactly the same as the relationship between arg and param in our example above  which I ll repeat here for symmetry   void foo int param      param    1     void bar       int arg   1    foo arg       It is exactly as if we had written the code this way      entering function  bar  here int arg   1     entering function  foo  here int param   arg  param    1     exiting function  foo  here    exiting function  bar  here   That is  the defining characteristic of what call by value means is that the callee  foo in this case  receives values as arguments  but has its own separate variables for those values from the variables of the caller  bar in this case    Going back to my metaphor above  if I m bar and you re foo  when I call you  I hand you a piece of paper with a value written on it  You call that piece of paper param  That value is a copy of the value I have written in my notebook  my local variables   in a variable I call arg    As an aside  depending on hardware and operating system  there are various calling conventions about how you call one function from another  The calling convention is like us deciding whether I write the value on a piece of my paper and then hand it to you  or if you have a piece of paper that I write it on  or if I write it on the wall in front of both of us  This is an interesting subject as well  but far beyond the scope of this already long answer    Call by reference  In call by reference  the function s formal parameters are simply new names for the same variables that the caller supplies as arguments   Going back to our example above  it s equivalent to      entering function  bar  here int arg   1     entering function  foo  here    aha  I note that  param  is just another name for  arg  arg    param       1     exiting function  foo  here    exiting function  bar  here   Since param is just another name for arg -- that is  they are the same variable  changes to param are reflected in arg  This is the fundamental way in which call by reference differs from call by value   Very few languages support call by reference  but C   can do it like this   void foo int amp  param      param    1     void bar       int arg   1    foo arg       In this case  param doesn t just have the same value as arg  it actually is arg  just by a different name  and so bar can observe that arg has been incremented   Note that this is not how any of Java  JavaScript  C  Objective-C  Python  or nearly any other popular language today works  This means that those languages are not call by reference  they are call by value   Addendum  call by object sharing  If what you have is call by value  but the actual value is a reference type or pointer type  then the  value  itself isn t very interesting  e g  in C it s just an integer of a platform-specific size  -- what s interesting is what that value points to   If what that reference type  that is  pointer  points to is mutable then an interesting effect is possible  you can modify the pointed-to value  and the caller can observe changes to the pointed-to value  even though the caller cannot observe changes to the pointer itself   To borrow the analogy of the URL again  the fact that I gave you a copy of the URL to a website is not particularly interesting if the thing we both care about is the website  not the URL  The fact that you scribbling over your copy of the URL doesn t affect my copy of the URL isn t a thing we care about  and in fact  in languages like Java and Python the  URL   or reference type value  can t be modified at all  only the thing pointed to by it can    Barbara Liskov  when she invented the CLU programming language  which had these semantics   realized that the existing terms  call by value  and  call by reference  weren t particularly useful for describing the semantics of this new language  So she invented a new term  call by object sharing   When discussing languages that are technically call by value  but where common types in use are reference or pointer types  that is  nearly every modern imperative  object-oriented  or multi-paradigm programming language   I find it s a lot less confusing to simply avoid talking about call by value or call by reference  Stick to call by object sharing  or simply call by object  and nobody will be confused   -

User · Answer

When passing by ref you are basically passing a pointer to the variable  Pass by value you are passing a copy of the variable   In basic usage this normally means pass by ref changes to the variable will seen be the calling method and pass by value they wont

User · Answer

It s a way how to pass arguments to functions  Passing by reference means the called functions  parameter will be the same as the callers  passed argument  not the value  but the identity - the variable itself   Pass by value means the called functions  parameter will be a copy of the callers  passed argument  The value will be the same  but the identity - the variable - is different  Thus changes to a parameter done by the called function in one case changes the argument passed and in the other case just changes the value of the parameter in the called function  which is only a copy   In a quick hurry    Java only supports pass by value  Always copies arguments  even though when copying a reference to an object  the parameter in the called function will point to the same object and changes to that object will be see in the caller  Since this can be confusing  here is what Jon Skeet has to say about this  C  supports pass by value and pass by reference  keyword ref used at caller and called function   Jon Skeet also has a nice explanation of this here  C   supports pass by value and pass by reference  reference parameter type used at called function   You will find an explanation of this below     Codes  Since my language is C    i will use that here     passes a pointer  called reference in java  to an integer void call by value int  p        1     p   NULL        passes an integer void call by value int p        2     p   42        passes an integer by reference void call by reference int  amp  p        3     p   42        this is the java style of passing references  NULL is called  null  there  void call by value special int  p        4      p   10     changes what p points to   what p references  in java         only changes the value of the parameter  but  not  of         the argument passed by the caller  thus it s pass-by-value      p   NULL     int main         int value   10      int   pointer    amp value       call by value pointer       1     assert pointer     amp value      pointer was copied      call by value value       2     assert value    10      value was copied      call by reference value       3     assert value    42      value was passed by reference      call by value special pointer       4        pointer was copied but what pointer references was changed      assert value    10  amp  amp  pointer     amp value       And an example in Java won t hurt   class Example       int value   0          similar to  4 case in the c   example     static void accept reference Example e        1         e value       will change the referenced object         e   null     will only change the parameter               similar to the  2 case in the c   example     static void accept primitive int v        2         v       will only change the parameter                    public static void main String    args            int value   0          Example ref   new Example       reference             note what we pass is the reference  not the object  we can t             pass objects  The reference is copied  pass-by-value           accept reference ref       1         assert ref    null  amp  amp  ref value    1              the primitive int variable is copied         accept primitive value       2         assert value    0            Wikipedia  http   en wikipedia org wiki Pass by reference Call by value  http   en wikipedia org wiki Pass by reference Call by reference  This guy pretty much nails it   http   javadude com articles passbyvalue htm

User · Answer

pass by value means how to pass value to a function by making use of arguments  in pass by value we copy the data stored in the variable we specify and it is slower than pass by reference bcse t he data is copied   of we make changes in the copied data the original data is not affected  nd in pass by refernce or pass by address we send direct link to the variable itself   or passing pointer to a variable  it is faster bcse less time is consumed

User · Answer

Here is an example      include  lt iostream gt   void by val int arg    arg    2    void by ref int amp arg    arg    2     int main         int x   0      by val x   std  cout  lt  lt  x  lt  lt  std  endl      prints 0     by ref x   std  cout  lt  lt  x  lt  lt  std  endl      prints 2      int y   0      by ref y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2     by val y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2

User · Answer

Here is an example      include  lt iostream gt   void by val int arg    arg    2    void by ref int amp arg    arg    2     int main         int x   0      by val x   std  cout  lt  lt  x  lt  lt  std  endl      prints 0     by ref x   std  cout  lt  lt  x  lt  lt  std  endl      prints 2      int y   0      by ref y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2     by val y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2

User · Answer

It s a way how to pass arguments to functions  Passing by reference means the called functions  parameter will be the same as the callers  passed argument  not the value  but the identity - the variable itself   Pass by value means the called functions  parameter will be a copy of the callers  passed argument  The value will be the same  but the identity - the variable - is different  Thus changes to a parameter done by the called function in one case changes the argument passed and in the other case just changes the value of the parameter in the called function  which is only a copy   In a quick hurry    Java only supports pass by value  Always copies arguments  even though when copying a reference to an object  the parameter in the called function will point to the same object and changes to that object will be see in the caller  Since this can be confusing  here is what Jon Skeet has to say about this  C  supports pass by value and pass by reference  keyword ref used at caller and called function   Jon Skeet also has a nice explanation of this here  C   supports pass by value and pass by reference  reference parameter type used at called function   You will find an explanation of this below     Codes  Since my language is C    i will use that here     passes a pointer  called reference in java  to an integer void call by value int  p        1     p   NULL        passes an integer void call by value int p        2     p   42        passes an integer by reference void call by reference int  amp  p        3     p   42        this is the java style of passing references  NULL is called  null  there  void call by value special int  p        4      p   10     changes what p points to   what p references  in java         only changes the value of the parameter  but  not  of         the argument passed by the caller  thus it s pass-by-value      p   NULL     int main         int value   10      int   pointer    amp value       call by value pointer       1     assert pointer     amp value      pointer was copied      call by value value       2     assert value    10      value was copied      call by reference value       3     assert value    42      value was passed by reference      call by value special pointer       4        pointer was copied but what pointer references was changed      assert value    10  amp  amp  pointer     amp value       And an example in Java won t hurt   class Example       int value   0          similar to  4 case in the c   example     static void accept reference Example e        1         e value       will change the referenced object         e   null     will only change the parameter               similar to the  2 case in the c   example     static void accept primitive int v        2         v       will only change the parameter                    public static void main String    args            int value   0          Example ref   new Example       reference             note what we pass is the reference  not the object  we can t             pass objects  The reference is copied  pass-by-value           accept reference ref       1         assert ref    null  amp  amp  ref value    1              the primitive int variable is copied         accept primitive value       2         assert value    0            Wikipedia  http   en wikipedia org wiki Pass by reference Call by value  http   en wikipedia org wiki Pass by reference Call by reference  This guy pretty much nails it   http   javadude com articles passbyvalue htm

User · Answer

Pass by value - The function copies the variable and works with a copy so it doesn t change anything in the original variable   Pass by reference - The function uses the original variable  if you change the variable in the other function  it changes in the original variable too   Example copy and use try this yourself and see      include  lt iostream gt   using namespace std   void funct1 int a     pass-by-value     a   6    now  a  is 6 only in funct1  but not in main or anywhere else   void funct2 int  amp a     pass-by-reference     a   7    now  a  is 7 both in funct2  main and everywhere else it ll be used    int main         int a   5       funct1 a       cout lt  lt endl lt  lt  A is currently   lt  lt a lt  lt endl lt  lt endl    will output 5     funct2 a       cout lt  lt endl lt  lt  A is currently   lt  lt a lt  lt endl lt  lt endl    will output 7      return 0      Keep it simple  peeps  Walls of text can be a bad habit

User · Answer

If you don t want to change the value of the original variable after passing it into a function  the function should be constructed with a  pass by value  parameter    Then the function will have ONLY the value but not the address of the passed in variable   Without the variable s address  the code inside the function cannot change the variable value as seen from the outside of the function     But if you want to give the function the ability to change the value of the variable as seen from the outside  you need to use pass by reference   As both the value and the address  reference  are passed in and available inside the function

User · Answer

First and foremost  the  quot pass by value vs  pass by reference quot  distinction as defined in the CS theory is now obsolete because the technique originally defined as  quot pass by reference quot  has since fallen out of favor and is seldom used now 1 Newer languages2 tend to use a different  but similar  pair of techniques to achieve the same effects  see below  which is the primary source of confusion  A secondary source of confusion is the fact that in  quot pass by reference quot    quot reference quot  has a narrower meaning than the general term  quot reference quot   because the phrase predates it    Now  the authentic definition is   When a parameter is passed by reference  the caller and the callee use the same variable for the parameter  If the callee modifies the parameter variable  the effect is visible to the caller s variable   When a parameter is passed by value  the caller and callee have two independent variables with the same value  If the callee modifies the parameter variable  the effect is not visible to the caller    Things to note in this definition are    quot Variable quot  here means the caller s  local or global  variable itself -- i e  if I pass a local variable by reference and assign to it  I ll change the caller s variable itself  not e g  whatever it is pointing to if it s a pointer   This is now considered bad practice  as an implicit dependency   As such  virtually all newer languages are exclusively  or almost exclusively pass-by-value  Pass-by-reference is now chiefly used in the form of  quot output inout arguments quot  in languages where a function cannot return more than one value    The meaning of  quot reference quot  in  quot pass by reference quot   The difference with the general  quot reference quot  term is that this  quot reference quot  is temporary and implicit  What the callee basically gets is a  quot variable quot  that is somehow  quot the same quot  as the original one  How specifically this effect is achieved is irrelevant  e g  the language may also expose some implementation details -- addresses  pointers  dereferencing -- this is all irrelevant  if the net effect is this  it s pass-by-reference      Now  in modern languages  variables tend to be of  quot reference types quot   another concept invented later than  quot pass by reference quot  and inspired by it   i e  the actual object data is stored separately somewhere  usually  on the heap   and only  quot references quot  to it are ever held in variables and passed as parameters 3 Passing such a reference falls under pass-by-value because a variable s value is technically the reference itself  not the referred object  However  the net effect on the program can be the same as either pass-by-value or pass-by-reference   If a reference is just taken from a caller s variable and passed as an argument  this has the same effect as pass-by-reference  if the referred object is mutated in the callee  the caller will see the change   However  if a variable holding this reference is reassigned  it will stop pointing to that object  so any further operations on this variable will instead affect whatever it is pointing to now    To have the same effect as pass-by-value  a copy of the object is made at some point  Options include   The caller can just make a private copy before the call and give the callee a reference to that instead  In some languages  some object types are  quot immutable quot   any operation on them that seems to alter the value actually creates a completely new object without affecting the original one  So  passing an object of such a type as an argument always has the effect of pass-by-value  a copy for the callee will be made automatically if and when it needs a change  and the caller s object will never be affected   In functional languages  all objects are immutable       As you may see  this pair of techniques is almost the same as those in the definition  only with a level of indirection  just replace  quot variable quot  with  quot referenced object quot   There s no agreed-upon name for them  which leads to contorted explanations like  quot call by value where the value is a reference quot   In 1975  Barbara Liskov suggested the term  quot call-by-object-sharing quot   or sometimes just  quot call-by-sharing quot   though it never quite caught on  Moreover  neither of these phrases draws a parallel with the original pair  No wonder the old terms ended up being reused in the absence of anything better  leading to confusion 4  NOTE  For a long time  this answer used to say   Say I want to share a web page with you  If I tell you the URL  I m passing by reference  You can use that URL to see the same web page I can see  If that page is changed  we both see the changes  If you delete the URL  all you re doing is destroying your reference to that page - you re not deleting the actual page itself  If I print out the page and give you the printout  I m passing by value  Your page is a disconnected copy of the original  You won t see any subsequent changes  and any changes that you make  e g  scribbling on your printout  will not show up on the original page  If you destroy the printout  you have actually destroyed your copy of the object - but the original web page remains intact   This is mostly correct except the narrower meaning of  quot reference quot  -- it being both temporary and implicit  it doesn t have to  but being explicit and or persistent are additional features  not a part of the pass-by-reference semantic  as explained above   A closer analogy would be giving you a copy of a document vs inviting you to work on the original   1Unless you are programming in Fortran or Visual Basic  it s not the default behavior  and in most languages in modern use  true call-by-reference is not even possible  2A fair amount of older ones support it  too 3In several modern languages  all types are reference types  This approach was pioneered by the language CLU in 1975 and has since been adopted by many other languages  including Python and Ruby  And many more languages use a hybrid approach  where some types are  quot value types quot  and others are  quot reference types quot  -- among them are C   Java  and JavaScript  4There s nothing bad with recycling a fitting old term per se  but one has to somehow make it clear which meaning is used each time  Not doing that is exactly what keeps causing confusion

User · Answer

Examples   class Dog     public      barkAt  const std  string amp  pOtherDog       const reference     barkAt  std  string pOtherDog       value      const  amp  is generally best  You don t incur the construction and destruction penalty  If the reference isn t const your interface is suggesting that it will change the passed in data

User · Answer

Here is an example      include  lt iostream gt   void by val int arg    arg    2    void by ref int amp arg    arg    2     int main         int x   0      by val x   std  cout  lt  lt  x  lt  lt  std  endl      prints 0     by ref x   std  cout  lt  lt  x  lt  lt  std  endl      prints 2      int y   0      by ref y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2     by val y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2

User · Answer

Here is an example that demonstrates the differences between pass by value - pointer value - reference   void swap by value int a  int b       int temp       temp   a      a   b      b   temp       void swap by pointer int  a  int  b       int temp       temp    a       a    b       b   temp        void swap by reference int  amp a  int  amp b       int temp       temp   a      a   b      b   temp     int main void       int arg1   1  arg2   2       swap by value arg1  arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 1 2      swap by pointer  amp arg1   amp arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 2 1      arg1   1                                  reset values     arg2   2      swap by reference arg1  arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 2 1     The    passing by reference    method has an important limitation  If a parameter is declared as passed by reference  so it is preceded by the  amp  sign  its corresponding actual parameter must be a variable   An actual parameter referring to    passed by value    formal parameter may be an expression in general  so it is allowed to use not only a variable but also a literal or even a function invocation s result   The function is not able to place a value in something other than a variable  It cannot assign a new value to a literal or force an expression to change its result   PS  You can also check Dylan Beattie answer in the current thread that explains it in plain words

User · Answer

Examples   class Dog     public      barkAt  const std  string amp  pOtherDog       const reference     barkAt  std  string pOtherDog       value      const  amp  is generally best  You don t incur the construction and destruction penalty  If the reference isn t const your interface is suggesting that it will change the passed in data

User · Answer

In short  Passed by value is WHAT it is and passed by reference is WHERE it is   If your value is VAR1    Happy Guy    you will only see  Happy Guy    If VAR1 changes to  Happy Gal    you won t know that  If it s passed by reference  and VAR1 changes  you will

User · Answer

Pass by value - The function copies the variable and works with a copy so it doesn t change anything in the original variable   Pass by reference - The function uses the original variable  if you change the variable in the other function  it changes in the original variable too   Example copy and use try this yourself and see      include  lt iostream gt   using namespace std   void funct1 int a     pass-by-value     a   6    now  a  is 6 only in funct1  but not in main or anywhere else   void funct2 int  amp a     pass-by-reference     a   7    now  a  is 7 both in funct2  main and everywhere else it ll be used    int main         int a   5       funct1 a       cout lt  lt endl lt  lt  A is currently   lt  lt a lt  lt endl lt  lt endl    will output 5     funct2 a       cout lt  lt endl lt  lt  A is currently   lt  lt a lt  lt endl lt  lt endl    will output 7      return 0      Keep it simple  peeps  Walls of text can be a bad habit

User · Answer

First and foremost  the  quot pass by value vs  pass by reference quot  distinction as defined in the CS theory is now obsolete because the technique originally defined as  quot pass by reference quot  has since fallen out of favor and is seldom used now 1 Newer languages2 tend to use a different  but similar  pair of techniques to achieve the same effects  see below  which is the primary source of confusion  A secondary source of confusion is the fact that in  quot pass by reference quot    quot reference quot  has a narrower meaning than the general term  quot reference quot   because the phrase predates it    Now  the authentic definition is   When a parameter is passed by reference  the caller and the callee use the same variable for the parameter  If the callee modifies the parameter variable  the effect is visible to the caller s variable   When a parameter is passed by value  the caller and callee have two independent variables with the same value  If the callee modifies the parameter variable  the effect is not visible to the caller    Things to note in this definition are    quot Variable quot  here means the caller s  local or global  variable itself -- i e  if I pass a local variable by reference and assign to it  I ll change the caller s variable itself  not e g  whatever it is pointing to if it s a pointer   This is now considered bad practice  as an implicit dependency   As such  virtually all newer languages are exclusively  or almost exclusively pass-by-value  Pass-by-reference is now chiefly used in the form of  quot output inout arguments quot  in languages where a function cannot return more than one value    The meaning of  quot reference quot  in  quot pass by reference quot   The difference with the general  quot reference quot  term is that this  quot reference quot  is temporary and implicit  What the callee basically gets is a  quot variable quot  that is somehow  quot the same quot  as the original one  How specifically this effect is achieved is irrelevant  e g  the language may also expose some implementation details -- addresses  pointers  dereferencing -- this is all irrelevant  if the net effect is this  it s pass-by-reference      Now  in modern languages  variables tend to be of  quot reference types quot   another concept invented later than  quot pass by reference quot  and inspired by it   i e  the actual object data is stored separately somewhere  usually  on the heap   and only  quot references quot  to it are ever held in variables and passed as parameters 3 Passing such a reference falls under pass-by-value because a variable s value is technically the reference itself  not the referred object  However  the net effect on the program can be the same as either pass-by-value or pass-by-reference   If a reference is just taken from a caller s variable and passed as an argument  this has the same effect as pass-by-reference  if the referred object is mutated in the callee  the caller will see the change   However  if a variable holding this reference is reassigned  it will stop pointing to that object  so any further operations on this variable will instead affect whatever it is pointing to now    To have the same effect as pass-by-value  a copy of the object is made at some point  Options include   The caller can just make a private copy before the call and give the callee a reference to that instead  In some languages  some object types are  quot immutable quot   any operation on them that seems to alter the value actually creates a completely new object without affecting the original one  So  passing an object of such a type as an argument always has the effect of pass-by-value  a copy for the callee will be made automatically if and when it needs a change  and the caller s object will never be affected   In functional languages  all objects are immutable       As you may see  this pair of techniques is almost the same as those in the definition  only with a level of indirection  just replace  quot variable quot  with  quot referenced object quot   There s no agreed-upon name for them  which leads to contorted explanations like  quot call by value where the value is a reference quot   In 1975  Barbara Liskov suggested the term  quot call-by-object-sharing quot   or sometimes just  quot call-by-sharing quot   though it never quite caught on  Moreover  neither of these phrases draws a parallel with the original pair  No wonder the old terms ended up being reused in the absence of anything better  leading to confusion 4  NOTE  For a long time  this answer used to say   Say I want to share a web page with you  If I tell you the URL  I m passing by reference  You can use that URL to see the same web page I can see  If that page is changed  we both see the changes  If you delete the URL  all you re doing is destroying your reference to that page - you re not deleting the actual page itself  If I print out the page and give you the printout  I m passing by value  Your page is a disconnected copy of the original  You won t see any subsequent changes  and any changes that you make  e g  scribbling on your printout  will not show up on the original page  If you destroy the printout  you have actually destroyed your copy of the object - but the original web page remains intact   This is mostly correct except the narrower meaning of  quot reference quot  -- it being both temporary and implicit  it doesn t have to  but being explicit and or persistent are additional features  not a part of the pass-by-reference semantic  as explained above   A closer analogy would be giving you a copy of a document vs inviting you to work on the original   1Unless you are programming in Fortran or Visual Basic  it s not the default behavior  and in most languages in modern use  true call-by-reference is not even possible  2A fair amount of older ones support it  too 3In several modern languages  all types are reference types  This approach was pioneered by the language CLU in 1975 and has since been adopted by many other languages  including Python and Ruby  And many more languages use a hybrid approach  where some types are  quot value types quot  and others are  quot reference types quot  -- among them are C   Java  and JavaScript  4There s nothing bad with recycling a fitting old term per se  but one has to somehow make it clear which meaning is used each time  Not doing that is exactly what keeps causing confusion

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Examples   class Dog     public      barkAt  const std  string amp  pOtherDog       const reference     barkAt  std  string pOtherDog       value      const  amp  is generally best  You don t incur the construction and destruction penalty  If the reference isn t const your interface is suggesting that it will change the passed in data

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1  Pass By Value   Call By Value    void printvalue int x               x   x   1          cout  lt  lt  x       6          int x   5     printvalue x      cout  lt  lt  x        5  In call by value  when you pass a value to printvalue x  i e  the argument which is 5  it is copied to void printvalue int x   Now  we have two different values 5 and the copied value 5 and these two values are stored in different memory locations  So if you make any change inside void printvalue int x  it won t reflect back to the argument  2  Pass By Reference  Call By Reference    void printvalue int  amp x              x   x   1         cout  lt  lt  x      6          int x   5     printvalue x      cout  lt  lt  x       6  In call by reference  there s only one difference  We use  amp  i e  the address operator  By doing void printvalue int  amp x  we are referring to the address of x which tells us that it both refers to the same location  Hence  any changes made inside the function will reflect outside  Now that you re here  you should also know about     3  Pass By Pointer  Call By Address    void printvalue int  x               x    x   1         cout  lt  lt   x      6          int x   5     printvalue  amp x      cout  lt  lt  x       6  In pass by address  the pointer int  x holds the address passed to it printvalue  amp x   Hence  any changes done inside the function will reflect outside

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Pass by value sends a COPY of the data stored in the variable you specify  pass by reference sends a direct link to the variable itself  So if you pass a variable by reference and then change the variable inside the block you passed it into  the original variable will be changed  If you simply pass by value  the original variable will not be able to be changed by the block you passed it into but you will get a copy of whatever it contained at the time of the call

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When passing by ref you are basically passing a pointer to the variable  Pass by value you are passing a copy of the variable   In basic usage this normally means pass by ref changes to the variable will seen be the calling method and pass by value they wont

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pass by value means how to pass value to a function by making use of arguments  in pass by value we copy the data stored in the variable we specify and it is slower than pass by reference bcse t he data is copied   of we make changes in the copied data the original data is not affected  nd in pass by refernce or pass by address we send direct link to the variable itself   or passing pointer to a variable  it is faster bcse less time is consumed

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When passing by ref you are basically passing a pointer to the variable  Pass by value you are passing a copy of the variable   In basic usage this normally means pass by ref changes to the variable will seen be the calling method and pass by value they wont

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The simplest way to get this is on an Excel file  Let   s say for example that you have two numbers  5 and 2 in cells A1 and B1 accordingly  and you want to find their sum in a third cell  let s say A2   You can do this in two ways    Either by passing their values to cell A2 by typing   5   2 into this cell  In this case  if the values of the cells A1 or B1 change  the sum in A2 remains the same  Or by passing the    references    of the cells A1 and B1 to cell A2 by typing   A1   B1  In this case  if the values of the cells A1 or B1 change  the sum in A2 changes too

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Before understanding the 2 terms  you MUST understand the following  Every object  has 2 things that can make it be distinguished    Its value  Its address    So if you say employee name    John   know that there are 2 things about name  Its value which is  John  and also its location in the memory which is some hexadecimal number maybe like  this  0x7fd5d258dd00    Depending on the language s architecture or the type  class  struct  etc   of your object  you would be either transferring  John  or 0x7fd5d258dd00  Passing  John  is known as passing by value  Passing 0x7fd5d258dd00 is known as passing by reference  Anyone who is pointing to this memory location will have access to the value of  John     For more on this  I recommend you to read about dereferencing a pointer and also why choose struct  value type  over class  reference type

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1  Pass By Value   Call By Value    void printvalue int x               x   x   1          cout  lt  lt  x       6          int x   5     printvalue x      cout  lt  lt  x        5  In call by value  when you pass a value to printvalue x  i e  the argument which is 5  it is copied to void printvalue int x   Now  we have two different values 5 and the copied value 5 and these two values are stored in different memory locations  So if you make any change inside void printvalue int x  it won t reflect back to the argument  2  Pass By Reference  Call By Reference    void printvalue int  amp x              x   x   1         cout  lt  lt  x      6          int x   5     printvalue x      cout  lt  lt  x       6  In call by reference  there s only one difference  We use  amp  i e  the address operator  By doing void printvalue int  amp x  we are referring to the address of x which tells us that it both refers to the same location  Hence  any changes made inside the function will reflect outside  Now that you re here  you should also know about     3  Pass By Pointer  Call By Address    void printvalue int  x               x    x   1         cout  lt  lt   x      6          int x   5     printvalue  amp x      cout  lt  lt  x       6  In pass by address  the pointer int  x holds the address passed to it printvalue  amp x   Hence  any changes done inside the function will reflect outside

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It s a way how to pass arguments to functions  Passing by reference means the called functions  parameter will be the same as the callers  passed argument  not the value  but the identity - the variable itself   Pass by value means the called functions  parameter will be a copy of the callers  passed argument  The value will be the same  but the identity - the variable - is different  Thus changes to a parameter done by the called function in one case changes the argument passed and in the other case just changes the value of the parameter in the called function  which is only a copy   In a quick hurry    Java only supports pass by value  Always copies arguments  even though when copying a reference to an object  the parameter in the called function will point to the same object and changes to that object will be see in the caller  Since this can be confusing  here is what Jon Skeet has to say about this  C  supports pass by value and pass by reference  keyword ref used at caller and called function   Jon Skeet also has a nice explanation of this here  C   supports pass by value and pass by reference  reference parameter type used at called function   You will find an explanation of this below     Codes  Since my language is C    i will use that here     passes a pointer  called reference in java  to an integer void call by value int  p        1     p   NULL        passes an integer void call by value int p        2     p   42        passes an integer by reference void call by reference int  amp  p        3     p   42        this is the java style of passing references  NULL is called  null  there  void call by value special int  p        4      p   10     changes what p points to   what p references  in java         only changes the value of the parameter  but  not  of         the argument passed by the caller  thus it s pass-by-value      p   NULL     int main         int value   10      int   pointer    amp value       call by value pointer       1     assert pointer     amp value      pointer was copied      call by value value       2     assert value    10      value was copied      call by reference value       3     assert value    42      value was passed by reference      call by value special pointer       4        pointer was copied but what pointer references was changed      assert value    10  amp  amp  pointer     amp value       And an example in Java won t hurt   class Example       int value   0          similar to  4 case in the c   example     static void accept reference Example e        1         e value       will change the referenced object         e   null     will only change the parameter               similar to the  2 case in the c   example     static void accept primitive int v        2         v       will only change the parameter                    public static void main String    args            int value   0          Example ref   new Example       reference             note what we pass is the reference  not the object  we can t             pass objects  The reference is copied  pass-by-value           accept reference ref       1         assert ref    null  amp  amp  ref value    1              the primitive int variable is copied         accept primitive value       2         assert value    0            Wikipedia  http   en wikipedia org wiki Pass by reference Call by value  http   en wikipedia org wiki Pass by reference Call by reference  This guy pretty much nails it   http   javadude com articles passbyvalue htm

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A major difference between them is that value-type variables store values  so specifying a value-type variable in a method call passes a copy of that variable s value to the method  Reference-type variables store references to objects  so specifying a reference-type variable as an argument passes the method a copy of the actual reference that refers to the object  Even though the reference itself is passed by value  the method can still use the reference it receives to interact with   and possibly modify   the original object  Similarly  when returning information from a method via a return statement  the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable  When a reference is returned  the calling method can use that reference to interact with the referenced object  So  in effect  objects are always passed by reference   In c   to pass a variable by reference so the called method can modify the variable s  C  provides keywords ref and out  Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference   the called method will be able to modify the original variable in the caller  The ref keyword is used for variables that already have been initialized in the calling method  Normally  when a method call contains an uninitialized variable as an argument  the compiler generates an error  Preceding a parameter with keyword out creates an output parameter  This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller  If the method does not assign a value to the output parameter in every possible path of execution  the compiler generates an error  This also prevents the compiler from generating an error message for an uninitialized variable that is passed as an argument to a method  A method can return only one value to its caller via a return statement  but can return many values by specifying multiple output  ref and or out  parameters   see c  discussion and examples here link text

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First and foremost  the  quot pass by value vs  pass by reference quot  distinction as defined in the CS theory is now obsolete because the technique originally defined as  quot pass by reference quot  has since fallen out of favor and is seldom used now 1 Newer languages2 tend to use a different  but similar  pair of techniques to achieve the same effects  see below  which is the primary source of confusion  A secondary source of confusion is the fact that in  quot pass by reference quot    quot reference quot  has a narrower meaning than the general term  quot reference quot   because the phrase predates it    Now  the authentic definition is   When a parameter is passed by reference  the caller and the callee use the same variable for the parameter  If the callee modifies the parameter variable  the effect is visible to the caller s variable   When a parameter is passed by value  the caller and callee have two independent variables with the same value  If the callee modifies the parameter variable  the effect is not visible to the caller    Things to note in this definition are    quot Variable quot  here means the caller s  local or global  variable itself -- i e  if I pass a local variable by reference and assign to it  I ll change the caller s variable itself  not e g  whatever it is pointing to if it s a pointer   This is now considered bad practice  as an implicit dependency   As such  virtually all newer languages are exclusively  or almost exclusively pass-by-value  Pass-by-reference is now chiefly used in the form of  quot output inout arguments quot  in languages where a function cannot return more than one value    The meaning of  quot reference quot  in  quot pass by reference quot   The difference with the general  quot reference quot  term is that this  quot reference quot  is temporary and implicit  What the callee basically gets is a  quot variable quot  that is somehow  quot the same quot  as the original one  How specifically this effect is achieved is irrelevant  e g  the language may also expose some implementation details -- addresses  pointers  dereferencing -- this is all irrelevant  if the net effect is this  it s pass-by-reference      Now  in modern languages  variables tend to be of  quot reference types quot   another concept invented later than  quot pass by reference quot  and inspired by it   i e  the actual object data is stored separately somewhere  usually  on the heap   and only  quot references quot  to it are ever held in variables and passed as parameters 3 Passing such a reference falls under pass-by-value because a variable s value is technically the reference itself  not the referred object  However  the net effect on the program can be the same as either pass-by-value or pass-by-reference   If a reference is just taken from a caller s variable and passed as an argument  this has the same effect as pass-by-reference  if the referred object is mutated in the callee  the caller will see the change   However  if a variable holding this reference is reassigned  it will stop pointing to that object  so any further operations on this variable will instead affect whatever it is pointing to now    To have the same effect as pass-by-value  a copy of the object is made at some point  Options include   The caller can just make a private copy before the call and give the callee a reference to that instead  In some languages  some object types are  quot immutable quot   any operation on them that seems to alter the value actually creates a completely new object without affecting the original one  So  passing an object of such a type as an argument always has the effect of pass-by-value  a copy for the callee will be made automatically if and when it needs a change  and the caller s object will never be affected   In functional languages  all objects are immutable       As you may see  this pair of techniques is almost the same as those in the definition  only with a level of indirection  just replace  quot variable quot  with  quot referenced object quot   There s no agreed-upon name for them  which leads to contorted explanations like  quot call by value where the value is a reference quot   In 1975  Barbara Liskov suggested the term  quot call-by-object-sharing quot   or sometimes just  quot call-by-sharing quot   though it never quite caught on  Moreover  neither of these phrases draws a parallel with the original pair  No wonder the old terms ended up being reused in the absence of anything better  leading to confusion 4  NOTE  For a long time  this answer used to say   Say I want to share a web page with you  If I tell you the URL  I m passing by reference  You can use that URL to see the same web page I can see  If that page is changed  we both see the changes  If you delete the URL  all you re doing is destroying your reference to that page - you re not deleting the actual page itself  If I print out the page and give you the printout  I m passing by value  Your page is a disconnected copy of the original  You won t see any subsequent changes  and any changes that you make  e g  scribbling on your printout  will not show up on the original page  If you destroy the printout  you have actually destroyed your copy of the object - but the original web page remains intact   This is mostly correct except the narrower meaning of  quot reference quot  -- it being both temporary and implicit  it doesn t have to  but being explicit and or persistent are additional features  not a part of the pass-by-reference semantic  as explained above   A closer analogy would be giving you a copy of a document vs inviting you to work on the original   1Unless you are programming in Fortran or Visual Basic  it s not the default behavior  and in most languages in modern use  true call-by-reference is not even possible  2A fair amount of older ones support it  too 3In several modern languages  all types are reference types  This approach was pioneered by the language CLU in 1975 and has since been adopted by many other languages  including Python and Ruby  And many more languages use a hybrid approach  where some types are  quot value types quot  and others are  quot reference types quot  -- among them are C   Java  and JavaScript  4There s nothing bad with recycling a fitting old term per se  but one has to somehow make it clear which meaning is used each time  Not doing that is exactly what keeps causing confusion

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Here is an example that demonstrates the differences between pass by value - pointer value - reference   void swap by value int a  int b       int temp       temp   a      a   b      b   temp       void swap by pointer int  a  int  b       int temp       temp    a       a    b       b   temp        void swap by reference int  amp a  int  amp b       int temp       temp   a      a   b      b   temp     int main void       int arg1   1  arg2   2       swap by value arg1  arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 1 2      swap by pointer  amp arg1   amp arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 2 1      arg1   1                                  reset values     arg2   2      swap by reference arg1  arg2       cout  lt  lt  arg1  lt  lt       lt  lt  arg2  lt  lt  endl       prints 2 1     The    passing by reference    method has an important limitation  If a parameter is declared as passed by reference  so it is preceded by the  amp  sign  its corresponding actual parameter must be a variable   An actual parameter referring to    passed by value    formal parameter may be an expression in general  so it is allowed to use not only a variable but also a literal or even a function invocation s result   The function is not able to place a value in something other than a variable  It cannot assign a new value to a literal or force an expression to change its result   PS  You can also check Dylan Beattie answer in the current thread that explains it in plain words

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A major difference between them is that value-type variables store values  so specifying a value-type variable in a method call passes a copy of that variable s value to the method  Reference-type variables store references to objects  so specifying a reference-type variable as an argument passes the method a copy of the actual reference that refers to the object  Even though the reference itself is passed by value  the method can still use the reference it receives to interact with   and possibly modify   the original object  Similarly  when returning information from a method via a return statement  the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable  When a reference is returned  the calling method can use that reference to interact with the referenced object  So  in effect  objects are always passed by reference   In c   to pass a variable by reference so the called method can modify the variable s  C  provides keywords ref and out  Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference   the called method will be able to modify the original variable in the caller  The ref keyword is used for variables that already have been initialized in the calling method  Normally  when a method call contains an uninitialized variable as an argument  the compiler generates an error  Preceding a parameter with keyword out creates an output parameter  This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller  If the method does not assign a value to the output parameter in every possible path of execution  the compiler generates an error  This also prevents the compiler from generating an error message for an uninitialized variable that is passed as an argument to a method  A method can return only one value to its caller via a return statement  but can return many values by specifying multiple output  ref and or out  parameters   see c  discussion and examples here link text

User · Answer

The simplest way to get this is on an Excel file  Let   s say for example that you have two numbers  5 and 2 in cells A1 and B1 accordingly  and you want to find their sum in a third cell  let s say A2   You can do this in two ways    Either by passing their values to cell A2 by typing   5   2 into this cell  In this case  if the values of the cells A1 or B1 change  the sum in A2 remains the same  Or by passing the    references    of the cells A1 and B1 to cell A2 by typing   A1   B1  In this case  if the values of the cells A1 or B1 change  the sum in A2 changes too

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When passing by ref you are basically passing a pointer to the variable  Pass by value you are passing a copy of the variable   In basic usage this normally means pass by ref changes to the variable will seen be the calling method and pass by value they wont

User · Answer

Before understanding the 2 terms  you MUST understand the following  Every object  has 2 things that can make it be distinguished    Its value  Its address    So if you say employee name    John   know that there are 2 things about name  Its value which is  John  and also its location in the memory which is some hexadecimal number maybe like  this  0x7fd5d258dd00    Depending on the language s architecture or the type  class  struct  etc   of your object  you would be either transferring  John  or 0x7fd5d258dd00  Passing  John  is known as passing by value  Passing 0x7fd5d258dd00 is known as passing by reference  Anyone who is pointing to this memory location will have access to the value of  John     For more on this  I recommend you to read about dereferencing a pointer and also why choose struct  value type  over class  reference type

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Examples   class Dog     public      barkAt  const std  string amp  pOtherDog       const reference     barkAt  std  string pOtherDog       value      const  amp  is generally best  You don t incur the construction and destruction penalty  If the reference isn t const your interface is suggesting that it will change the passed in data

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Pass by value sends a COPY of the data stored in the variable you specify  pass by reference sends a direct link to the variable itself  So if you pass a variable by reference and then change the variable inside the block you passed it into  the original variable will be changed  If you simply pass by value  the original variable will not be able to be changed by the block you passed it into but you will get a copy of whatever it contained at the time of the call

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First and foremost  the  quot pass by value vs  pass by reference quot  distinction as defined in the CS theory is now obsolete because the technique originally defined as  quot pass by reference quot  has since fallen out of favor and is seldom used now 1 Newer languages2 tend to use a different  but similar  pair of techniques to achieve the same effects  see below  which is the primary source of confusion  A secondary source of confusion is the fact that in  quot pass by reference quot    quot reference quot  has a narrower meaning than the general term  quot reference quot   because the phrase predates it    Now  the authentic definition is   When a parameter is passed by reference  the caller and the callee use the same variable for the parameter  If the callee modifies the parameter variable  the effect is visible to the caller s variable   When a parameter is passed by value  the caller and callee have two independent variables with the same value  If the callee modifies the parameter variable  the effect is not visible to the caller    Things to note in this definition are    quot Variable quot  here means the caller s  local or global  variable itself -- i e  if I pass a local variable by reference and assign to it  I ll change the caller s variable itself  not e g  whatever it is pointing to if it s a pointer   This is now considered bad practice  as an implicit dependency   As such  virtually all newer languages are exclusively  or almost exclusively pass-by-value  Pass-by-reference is now chiefly used in the form of  quot output inout arguments quot  in languages where a function cannot return more than one value    The meaning of  quot reference quot  in  quot pass by reference quot   The difference with the general  quot reference quot  term is that this  quot reference quot  is temporary and implicit  What the callee basically gets is a  quot variable quot  that is somehow  quot the same quot  as the original one  How specifically this effect is achieved is irrelevant  e g  the language may also expose some implementation details -- addresses  pointers  dereferencing -- this is all irrelevant  if the net effect is this  it s pass-by-reference      Now  in modern languages  variables tend to be of  quot reference types quot   another concept invented later than  quot pass by reference quot  and inspired by it   i e  the actual object data is stored separately somewhere  usually  on the heap   and only  quot references quot  to it are ever held in variables and passed as parameters 3 Passing such a reference falls under pass-by-value because a variable s value is technically the reference itself  not the referred object  However  the net effect on the program can be the same as either pass-by-value or pass-by-reference   If a reference is just taken from a caller s variable and passed as an argument  this has the same effect as pass-by-reference  if the referred object is mutated in the callee  the caller will see the change   However  if a variable holding this reference is reassigned  it will stop pointing to that object  so any further operations on this variable will instead affect whatever it is pointing to now    To have the same effect as pass-by-value  a copy of the object is made at some point  Options include   The caller can just make a private copy before the call and give the callee a reference to that instead  In some languages  some object types are  quot immutable quot   any operation on them that seems to alter the value actually creates a completely new object without affecting the original one  So  passing an object of such a type as an argument always has the effect of pass-by-value  a copy for the callee will be made automatically if and when it needs a change  and the caller s object will never be affected   In functional languages  all objects are immutable       As you may see  this pair of techniques is almost the same as those in the definition  only with a level of indirection  just replace  quot variable quot  with  quot referenced object quot   There s no agreed-upon name for them  which leads to contorted explanations like  quot call by value where the value is a reference quot   In 1975  Barbara Liskov suggested the term  quot call-by-object-sharing quot   or sometimes just  quot call-by-sharing quot   though it never quite caught on  Moreover  neither of these phrases draws a parallel with the original pair  No wonder the old terms ended up being reused in the absence of anything better  leading to confusion 4  NOTE  For a long time  this answer used to say   Say I want to share a web page with you  If I tell you the URL  I m passing by reference  You can use that URL to see the same web page I can see  If that page is changed  we both see the changes  If you delete the URL  all you re doing is destroying your reference to that page - you re not deleting the actual page itself  If I print out the page and give you the printout  I m passing by value  Your page is a disconnected copy of the original  You won t see any subsequent changes  and any changes that you make  e g  scribbling on your printout  will not show up on the original page  If you destroy the printout  you have actually destroyed your copy of the object - but the original web page remains intact   This is mostly correct except the narrower meaning of  quot reference quot  -- it being both temporary and implicit  it doesn t have to  but being explicit and or persistent are additional features  not a part of the pass-by-reference semantic  as explained above   A closer analogy would be giving you a copy of a document vs inviting you to work on the original   1Unless you are programming in Fortran or Visual Basic  it s not the default behavior  and in most languages in modern use  true call-by-reference is not even possible  2A fair amount of older ones support it  too 3In several modern languages  all types are reference types  This approach was pioneered by the language CLU in 1975 and has since been adopted by many other languages  including Python and Ruby  And many more languages use a hybrid approach  where some types are  quot value types quot  and others are  quot reference types quot  -- among them are C   Java  and JavaScript  4There s nothing bad with recycling a fitting old term per se  but one has to somehow make it clear which meaning is used each time  Not doing that is exactly what keeps causing confusion

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Here is an example      include  lt iostream gt   void by val int arg    arg    2    void by ref int amp arg    arg    2     int main         int x   0      by val x   std  cout  lt  lt  x  lt  lt  std  endl      prints 0     by ref x   std  cout  lt  lt  x  lt  lt  std  endl      prints 2      int y   0      by ref y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2     by val y   std  cout  lt  lt  y  lt  lt  std  endl      prints 2

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A major difference between them is that value-type variables store values  so specifying a value-type variable in a method call passes a copy of that variable s value to the method  Reference-type variables store references to objects  so specifying a reference-type variable as an argument passes the method a copy of the actual reference that refers to the object  Even though the reference itself is passed by value  the method can still use the reference it receives to interact with   and possibly modify   the original object  Similarly  when returning information from a method via a return statement  the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable  When a reference is returned  the calling method can use that reference to interact with the referenced object  So  in effect  objects are always passed by reference   In c   to pass a variable by reference so the called method can modify the variable s  C  provides keywords ref and out  Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference   the called method will be able to modify the original variable in the caller  The ref keyword is used for variables that already have been initialized in the calling method  Normally  when a method call contains an uninitialized variable as an argument  the compiler generates an error  Preceding a parameter with keyword out creates an output parameter  This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller  If the method does not assign a value to the output parameter in every possible path of execution  the compiler generates an error  This also prevents the compiler from generating an error message for an uninitialized variable that is passed as an argument to a method  A method can return only one value to its caller via a return statement  but can return many values by specifying multiple output  ref and or out  parameters   see c  discussion and examples here link text

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Many answers here  and in particular the most highly upvoted answer  are factually incorrect  since they misunderstand what  call by reference  really means  Here s my attempt to set matters straight   TL DR  In simplest terms    call by value means that you pass values as function arguments call by reference means that you pass variables as function arguments   In metaphoric terms    Call by value is where I write down something on a piece of paper and hand it to you  Maybe it s a URL  maybe it s a complete copy of War and Peace  No matter what it is  it s on a piece of paper which I ve given to you  and so now it is effectively your piece of paper  You are now free to scribble on that piece of paper  or use that piece of paper to find something somewhere else and fiddle with it  whatever  Call by reference is when I give you my notebook which has something written down in it  You may scribble in my notebook  maybe I want you to  maybe I don t   and afterwards I keep my notebook  with whatever scribbles you ve put there  Also  if what either you or I wrote there is information about how to find something somewhere else  either you or I can go there and fiddle with that information    What  call by value  and  call by reference  don t mean  Note that both of these concepts are completely independent and orthogonal from the concept of reference types  which in Java is all types that are subtypes of Object  and in C  all class types   or the concept of pointer types like in C  which are semantically equivalent to Java s  reference types   simply with different syntax    The notion of reference type corresponds to a URL  it is both itself a piece of information  and it is a reference  a pointer  if you will  to other information  You can have many copies of a URL in different places  and they don t change what website they all link to  if the website is updated then every URL copy will still lead to the updated information  Conversely  changing the URL in any one place won t affect any other written copy of the URL   Note that C   has a notion of  references   e g  int amp   that is not like Java and C  s  reference types   but is like  call by reference   Java and C  s  reference types   and all types in Python  are like what C and C   call  pointer types   e g  int       OK  here s the longer and more formal explanation   Terminology  To start with  I want to highlight some important bits of terminology  to help clarify my answer and to ensure we re all referring to the same ideas when we are using words   In practice  I believe the vast majority of confusion about topics such as these stems from using words in ways that to not fully communicate the meaning that was intended    To start  here s an example in some C-like language of a function declaration   void foo int param        line 1   param    1      And here s an example of calling this function   void bar       int arg   1      line 2   foo arg          line 3     Using this example  I want to define some important bits of terminology    foo is a function declared on line 1  Java insists on making all functions methods  but the concept is the same without loss of generality  C and C   make a distinction between declaration and definition which I won t go into here  param is a formal parameter to foo  also declared on line 1 arg is a variable  specifically a local variable of the function bar  declared and initialized on line 2 arg is also an argument to a specific invocation of foo on line 3   There are two very important sets of concepts to distinguish here  The first is value versus variable    A value is the result of evaluating an expression in the language  For example  in the bar function above  after the line int arg   1   the expression arg has the value 1  A variable is a container for values  A variable can be mutable  this is the default in most C-like languages   read-only  e g  declared using Java s final or C  s readonly  or deeply immutable  e g  using C   s const     The other important pair of concepts to distinguish is parameter versus argument    A parameter  also called a formal parameter  is a variable which must be supplied by the caller when calling a function  An argument is a value that is supplied by the caller of a function to satisfy a specific formal parameter of that function   Call by value  In call by value  the function s formal parameters are variables that are newly created for the function invocation  and which are initialized with the values of their arguments   This works exactly the same way that any other kinds of variables are initialized with values  For example   int arg   1  int another variable   arg    Here arg and another variable are completely independent variables -- their values can change independently of each other  However  at the point where another variable is declared  it is initialized to hold the same value that arg holds -- which is 1   Since they are independent variables  changes to another variable do not affect arg   int arg   1  int another variable   arg  another variable   2   assert arg    1     true assert another variable    2     true   This is exactly the same as the relationship between arg and param in our example above  which I ll repeat here for symmetry   void foo int param      param    1     void bar       int arg   1    foo arg       It is exactly as if we had written the code this way      entering function  bar  here int arg   1     entering function  foo  here int param   arg  param    1     exiting function  foo  here    exiting function  bar  here   That is  the defining characteristic of what call by value means is that the callee  foo in this case  receives values as arguments  but has its own separate variables for those values from the variables of the caller  bar in this case    Going back to my metaphor above  if I m bar and you re foo  when I call you  I hand you a piece of paper with a value written on it  You call that piece of paper param  That value is a copy of the value I have written in my notebook  my local variables   in a variable I call arg    As an aside  depending on hardware and operating system  there are various calling conventions about how you call one function from another  The calling convention is like us deciding whether I write the value on a piece of my paper and then hand it to you  or if you have a piece of paper that I write it on  or if I write it on the wall in front of both of us  This is an interesting subject as well  but far beyond the scope of this already long answer    Call by reference  In call by reference  the function s formal parameters are simply new names for the same variables that the caller supplies as arguments   Going back to our example above  it s equivalent to      entering function  bar  here int arg   1     entering function  foo  here    aha  I note that  param  is just another name for  arg  arg    param       1     exiting function  foo  here    exiting function  bar  here   Since param is just another name for arg -- that is  they are the same variable  changes to param are reflected in arg  This is the fundamental way in which call by reference differs from call by value   Very few languages support call by reference  but C   can do it like this   void foo int amp  param      param    1     void bar       int arg   1    foo arg       In this case  param doesn t just have the same value as arg  it actually is arg  just by a different name  and so bar can observe that arg has been incremented   Note that this is not how any of Java  JavaScript  C  Objective-C  Python  or nearly any other popular language today works  This means that those languages are not call by reference  they are call by value   Addendum  call by object sharing  If what you have is call by value  but the actual value is a reference type or pointer type  then the  value  itself isn t very interesting  e g  in C it s just an integer of a platform-specific size  -- what s interesting is what that value points to   If what that reference type  that is  pointer  points to is mutable then an interesting effect is possible  you can modify the pointed-to value  and the caller can observe changes to the pointed-to value  even though the caller cannot observe changes to the pointer itself   To borrow the analogy of the URL again  the fact that I gave you a copy of the URL to a website is not particularly interesting if the thing we both care about is the website  not the URL  The fact that you scribbling over your copy of the URL doesn t affect my copy of the URL isn t a thing we care about  and in fact  in languages like Java and Python the  URL   or reference type value  can t be modified at all  only the thing pointed to by it can    Barbara Liskov  when she invented the CLU programming language  which had these semantics   realized that the existing terms  call by value  and  call by reference  weren t particularly useful for describing the semantics of this new language  So she invented a new term  call by object sharing   When discussing languages that are technically call by value  but where common types in use are reference or pointer types  that is  nearly every modern imperative  object-oriented  or multi-paradigm programming language   I find it s a lot less confusing to simply avoid talking about call by value or call by reference  Stick to call by object sharing  or simply call by object  and nobody will be confused   -

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Pass by value sends a COPY of the data stored in the variable you specify  pass by reference sends a direct link to the variable itself  So if you pass a variable by reference and then change the variable inside the block you passed it into  the original variable will be changed  If you simply pass by value  the original variable will not be able to be changed by the block you passed it into but you will get a copy of whatever it contained at the time of the call

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It s a way how to pass arguments to functions  Passing by reference means the called functions  parameter will be the same as the callers  passed argument  not the value  but the identity - the variable itself   Pass by value means the called functions  parameter will be a copy of the callers  passed argument  The value will be the same  but the identity - the variable - is different  Thus changes to a parameter done by the called function in one case changes the argument passed and in the other case just changes the value of the parameter in the called function  which is only a copy   In a quick hurry    Java only supports pass by value  Always copies arguments  even though when copying a reference to an object  the parameter in the called function will point to the same object and changes to that object will be see in the caller  Since this can be confusing  here is what Jon Skeet has to say about this  C  supports pass by value and pass by reference  keyword ref used at caller and called function   Jon Skeet also has a nice explanation of this here  C   supports pass by value and pass by reference  reference parameter type used at called function   You will find an explanation of this below     Codes  Since my language is C    i will use that here     passes a pointer  called reference in java  to an integer void call by value int  p        1     p   NULL        passes an integer void call by value int p        2     p   42        passes an integer by reference void call by reference int  amp  p        3     p   42        this is the java style of passing references  NULL is called  null  there  void call by value special int  p        4      p   10     changes what p points to   what p references  in java         only changes the value of the parameter  but  not  of         the argument passed by the caller  thus it s pass-by-value      p   NULL     int main         int value   10      int   pointer    amp value       call by value pointer       1     assert pointer     amp value      pointer was copied      call by value value       2     assert value    10      value was copied      call by reference value       3     assert value    42      value was passed by reference      call by value special pointer       4        pointer was copied but what pointer references was changed      assert value    10  amp  amp  pointer     amp value       And an example in Java won t hurt   class Example       int value   0          similar to  4 case in the c   example     static void accept reference Example e        1         e value       will change the referenced object         e   null     will only change the parameter               similar to the  2 case in the c   example     static void accept primitive int v        2         v       will only change the parameter                    public static void main String    args            int value   0          Example ref   new Example       reference             note what we pass is the reference  not the object  we can t             pass objects  The reference is copied  pass-by-value           accept reference ref       1         assert ref    null  amp  amp  ref value    1              the primitive int variable is copied         accept primitive value       2         assert value    0            Wikipedia  http   en wikipedia org wiki Pass by reference Call by value  http   en wikipedia org wiki Pass by reference Call by reference  This guy pretty much nails it   http   javadude com articles passbyvalue htm

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If you don t want to change the value of the original variable after passing it into a function  the function should be constructed with a  pass by value  parameter    Then the function will have ONLY the value but not the address of the passed in variable   Without the variable s address  the code inside the function cannot change the variable value as seen from the outside of the function     But if you want to give the function the ability to change the value of the variable as seen from the outside  you need to use pass by reference   As both the value and the address  reference  are passed in and available inside the function

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In short  Passed by value is WHAT it is and passed by reference is WHERE it is   If your value is VAR1    Happy Guy    you will only see  Happy Guy    If VAR1 changes to  Happy Gal    you won t know that  If it s passed by reference  and VAR1 changes  you will

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A major difference between them is that value-type variables store values  so specifying a value-type variable in a method call passes a copy of that variable s value to the method  Reference-type variables store references to objects  so specifying a reference-type variable as an argument passes the method a copy of the actual reference that refers to the object  Even though the reference itself is passed by value  the method can still use the reference it receives to interact with   and possibly modify   the original object  Similarly  when returning information from a method via a return statement  the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable  When a reference is returned  the calling method can use that reference to interact with the referenced object  So  in effect  objects are always passed by reference   In c   to pass a variable by reference so the called method can modify the variable s  C  provides keywords ref and out  Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference   the called method will be able to modify the original variable in the caller  The ref keyword is used for variables that already have been initialized in the calling method  Normally  when a method call contains an uninitialized variable as an argument  the compiler generates an error  Preceding a parameter with keyword out creates an output parameter  This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller  If the method does not assign a value to the output parameter in every possible path of execution  the compiler generates an error  This also prevents the compiler from generating an error message for an uninitialized variable that is passed as an argument to a method  A method can return only one value to its caller via a return statement  but can return many values by specifying multiple output  ref and or out  parameters   see c  discussion and examples here link text

[language-agnostic] What's the difference between passing by reference vs. passing by value?

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