What are the differences between a pointer variable and a reference variable in C

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I know references are syntactic sugar  so code is easier to read and write   But what are the differences

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It doesn t matter how much space it takes up since you can t actually see any side effect  without executing code  of whatever space it would take up   On the other hand  one major difference between references and pointers is that temporaries assigned to const references live until the const reference goes out of scope   For example   class scope test   public       scope test     printf  scope test done  n                     const scope test  amp test  scope test        printf  in scope n        will print   in scope scope test done    This is the language mechanism that allows ScopeGuard to work

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I always decide by this rule from C   Core Guidelines      Prefer T  over T amp  when  no argument  is a valid option

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A reference is a const pointer  int   const a    amp b is the same as int amp  a   b  This is why there s is no such thing as a const reference  because it is already const  whereas a reference to const is const int   const a  When you compile using -O0  the compiler will place the address of b on the stack in both situations  and as a member of a class  it will also be present in the object on the stack heap identically to if you had declared a const pointer  With -Ofast  it is free to optimise this out    Unlike a const pointer  there is no way to take the address of the reference itself  as it will be interpreted as the address of the variable it references  Because of this  a reference will always be optimised away  but if the program absolutely needs the address of a const pointer on -Ofast i e  you print the address  then the const pointer will be placed on the stack    include  lt iostream gt   int main       int a  1    int  b    amp a    std  cout  lt  lt  b      int main       int a  1    int amp  b   a    std  cout  lt  lt   amp b      they both have the same assembly output -Ofast  main          sub     rsp  24         mov     edi  OFFSET FLAT  ZSt4cout         lea     rsi   rsp 12          mov     DWORD PTR  rsp 12   1         call    std  basic ostream lt char  std  char traits lt char gt   gt  amp  std  basic ostream lt char  std  char traits lt char gt   gt    M insert lt void const  gt  void const           xor     eax  eax         add     rsp  24         ret  -O0  main          push    rbp         mov     rbp  rsp         sub     rsp  16         mov     DWORD PTR  rbp-12   1         lea     rax   rbp-12          mov     QWORD PTR  rbp-8   rax         mov     rax  QWORD PTR  rbp-8          mov     rsi  rax         mov     edi  OFFSET FLAT  ZSt4cout         call    std  basic ostream lt char  std  char traits lt char gt   gt   operator lt  lt  void const           mov     eax  0         leave         ret   As members of an object they are identical on -O0 through -Ofast     include  lt iostream gt  int b 1  struct A  int  i  amp b  int amp  j b    A a  int main       std  cout  lt  lt   amp a j  lt  lt   amp a i     The address of b is stored twice in the object    a           quad   b          quad   b          mov     rax  QWORD PTR a rip 8     amp a j         mov     esi  OFFSET FLAT a    amp a i

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A reference is not another name given to some memory  It s a immutable pointer that is automatically de-referenced on usage  Basically it boils down to   int amp  j   i    It internally becomes  int  const j    amp i

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While both references and pointers are used to indirectly access another value  there are two important differences between references and pointers  The first is that a reference always refers to an object  It is an error to define a reference without initializing it  The behavior of assignment is the second important difference  Assigning to a reference changes the object to which the reference is bound  it does not rebind the reference to another object  Once initialized  a reference always refers to the same underlying object   Consider these two program fragments  In the first  we assign one pointer to another   int ival   1024  ival2   2048  int  pi    amp ival   pi2    amp ival2  pi   pi2        pi now points to ival2   After the assignment  ival  the object addressed by pi remains unchanged  The assignment changes the value of pi  making it point to a different object  Now consider a similar program that assigns two references   int  amp ri   ival   amp ri2   ival2  ri   ri2        assigns ival2 to ival   This assignment changes ival  the value referenced by ri  and not the reference itself  After the assignment  the two references still refer to their original objects  and the value of those objects is now the same as well

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Actually  a reference is not really like a pointer   A compiler keeps  references  to variables  associating a name with a memory address  that s its job to translate any variable name to a memory address when compiling   When you create a reference  you only tell the compiler that you assign another name to the pointer variable  that s why references cannot  point to null   because a variable cannot be  and not be   Pointers are variables  they contain the address of some other variable  or can be null  The important thing is that a pointer has a value  while a reference only has a variable that it is referencing   Now some explanation of real code   int a   0  int amp  b   a    Here you are not creating another variable that points to a  you are just adding another name to the memory content holding the value of a  This memory now has two names  a and b  and it can be addressed using either name   void increment int amp  n        n   n   1     int a  increment a     When calling a function  the compiler usually generates memory spaces for the arguments to be copied to  The function signature defines the spaces that should be created and gives the name that should be used for these spaces  Declaring a parameter as a reference just tells the compiler to use the input variable memory space instead of allocating a new memory space during the method call  It may seem strange to say that your function will be directly manipulating a variable declared in the calling scope  but remember that when executing compiled code  there is no more scope  there is just plain flat memory  and your function code could manipulate any variables   Now there may be some cases where your compiler may not be able to know the reference when compiling  like when using an extern variable  So a reference may or may not be implemented as a pointer in the underlying code  But in the examples I gave you  it will most likely not be implemented with a pointer

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Taryn  said      You can t take the address of a reference like you can with pointers    Actually you can   I m quoting from an answer on another question      The C   FAQ says it best       Unlike a pointer  once a reference is bound to an object  it can not be  reseated  to another object  The reference itself isn t an object  it has no identity  taking the address of a reference gives you the address of the referent  remember  the reference is its referent

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The direct answer  What is a reference in C    Some specific instance of type that is not an object type   What is a pointer in C    Some specific instance of type that is an object type   From the ISO C   definition of object type      An object type is a  possibly cv-qualified  type that is not a function type  not a reference type  and not cv void    It may be important to know  object type is a top-level category of the type universe in C    Reference is also a top-level category  But pointer is not   Pointers and references are mentioned together in the context of compound type  This is basically due to the nature of the declarator syntax inherited from  and extended  C  which has no references   Besides  there are more than one kind of declarator of references since C   11  while pointers are still  unityped    amp   amp  amp  vs      So drafting a language specific by  extension  with similar style of C in this context is somewhat reasonable   I will still argue that the syntax of declarators wastes the syntactic expressiveness a lot  makes both human users and implementations frustrating  Thus  all of them are not qualified to be built-in in a new language design  This is a totally different topic about PL design  though    Otherwise  it is insignificant that pointers can be qualified as a specific sorts of types with references together  They simply share too few common properties besides the syntax similarity  so there is no need to put them together in most cases   Note the statements above only mentions  pointers  and  references  as types  There are some interested questions about their instances  like variables   There also come too many misconceptions   The differences of the top-level categories can already reveal many concrete differences not tied to pointers directly    Object types can have top-level cv qualifiers  References cannot  Variable of object types do occupy storage as per the abstract machine semantics  Reference do not necessary occupy storage  see the section about misconceptions below for details         A few more special rules on references    Compound declarators are more restrictive on references  References can collapse    Special rules on  amp  amp  parameters  as the  forwarding references   based on reference collapsing during template parameter deduction allow  perfect forwarding  of parameters   References have special rules in initialization  The lifetime of variable declared as a reference type can be different to ordinary objects via extension    BTW  a few other contexts like initialization involving std  initializer list follows some similar rules of reference lifetime extension  It is another can of worms         The misconceptions  Syntactic sugar     I know references are syntactic sugar  so code is easier to read and write    Technically  this is plain wrong  References are not syntactic sugar of any other features in C    because they cannot be exactly replaced by other features without any semantic differences    Similarly  lambda-expressions are not syntactic sugar of any other features in C   because it cannot be precisely simulated with  unspecified  properties like the declaration order of the captured variables  which may be important because the initialization order of such variables can be significant    C   only has a few kinds of syntactic sugars in this strict sense  One instance is  inherited from C  the built-in  non-overloaded  operator     which is defined exactly having same semantic properties of specific forms of combination over built-in operator unary   and binary     Storage     So  a pointer and a reference both use the same amount of memory    The statement above is simply wrong  To avoid such misconceptions  look at the ISO C   rules instead   From  intro object  1          An object occupies a region of storage in its period of construction  throughout its lifetime  and in its period of destruction        From  dcl ref  4      It is unspecified whether or not a reference requires storage    Note these are semantic properties   Pragmatics  Even that pointers are not qualified enough to be put together with references in the sense of the language design  there are still some arguments making it debatable to make choice between them in some other contexts  for example  when making choices on parameter types   But this is not the whole story  I mean  there are more things than pointers vs references you have to consider   If you don t have to stick on such over-specific choices  in most cases the answer is short  you do not have the necessity to use pointers  so you don t  Pointers are usually bad enough because they imply too many things you don t expect and they will rely on too many implicit assumptions undermining the maintainability and  even  portability of the code  Unnecessarily relying on pointers is definitely a bad style and it should be avoided in the sense of modern C    Reconsider your purpose and you will finally find that pointer is the feature of last sorts in most cases    Sometimes the language rules explicitly require specific types to be used  If you want to use these features  obey the rules    Copy constructors require specific types of cv- amp  reference type as the 1st parameter type   And usually it should be const qualified   Move constructors require specific types of cv- amp  amp  reference type as the 1st parameter type   And usually there should be no qualifiers   Specific overloads of operators require reference or non reference types  For example    Overloaded operator  as special member functions requires reference types similar to 1st parameter of copy move constructors  Postfix    requires dummy int        If you know pass-by-value  i e  using non-reference types  is sufficient  use it directly  particularly when using an implementation supporting C  17 mandated copy elision   Warning  However  to exhaustively reason about the necessity can be very complicated   If you want to operate some handles with ownership  use smart pointers like unique ptr and shared ptr  or even with homebrew ones by yourself if you require them to be opaque   rather than raw pointers  If you are doing some iterations over a range  use iterators  or some ranges which are not provided by the standard library yet   rather than raw pointers unless you are convinced raw pointers will do better  e g  for less header dependencies  in very specific cases  If you know pass-by-value is sufficient and you want some explicit nullable semantics  use wrapper like std  optional  rather than raw pointers  If you know pass-by-value is not ideal for the reasons above  and you don t want nullable semantics  use  lvalue  rvalue  forwarding -references  Even when you do want semantics like traditional pointer  there are often something more appropriate  like observer ptr in Library Fundamental TS    The only exceptions cannot be worked around in the current language    When you are implementing smart pointers above  you may have to deal with raw pointers  Specific language-interoperation routines require pointers  like operator new   However  cv-void  is still quite different and safer compared to the ordinary object pointers because it rules out unexpected pointer arithmetics unless you are relying on some non conforming extension on void  like GNU s   Function pointers can be converted from lambda expressions without captures  while function references cannot  You have to use function pointers in non-generic code for such cases  even you deliberately do not want nullable values    So  in practice  the answer is so obvious  when in doubt  avoid pointers  You have to use pointers only when there are very explicit reasons that nothing else is more appropriate  Except a few exceptional cases mentioned above  such choices are almost always not purely C  -specific  but likely to be language-implementation-specific   Such instances can be    You have to serve to old-style  C  APIs  You have to meet the ABI requirements of specific C   implementations  You have to interoperate at runtime with different language implementations  including various assemblies  language runtime and FFI of some high-level client languages  based on assumptions of specific implementations  You have to improve efficiency of the translation  compilation  amp  linking  in some extreme cases  You have to avoid symbol bloat in some extreme cases    Language neutrality caveats  If you come to see the question via some Google search result  not specific to C     this is very likely to be the wrong place   References in C   is quite  odd   as it is essentially not first-class  they will be treated as the objects or the functions being referred to so they have no chance to support some first-class operations like being the left operand of the member access operator independently to the type of the referred object  Other languages may or may not have similar restrictions on their references   References in C   will likely not preserve the meaning across different languages  For example  references in general do not imply nonnull properties on values like they in C    so such assumptions may not work in some other languages  and you will find counterexamples quite easily  e g  Java  C           There can still be some common properties among references in different programming languages in general  but let s leave it for some other questions in SO    A side note  the question may be significant earlier than any  C-like  languages are involved  like ALGOL 68 vs  PL I

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I use references unless I need either of these    Null pointers can be used as a sentinel value  often a cheap way to avoid function overloading or use of a bool  You can do arithmetic on a pointer  For example  p    offset

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References are very similar to pointers  but they are specifically crafted to be helpful to optimizing compilers    References are designed such that it is substantially easier for the compiler to trace which reference aliases which variables  Two major features are very important  no  reference arithmetic  and no reassigning of references  These allow the compiler to figure out which references alias which variables at compile time  References are allowed to refer to variables which do not have memory addresses  such as those the compiler chooses to put into registers  If you take the address of a local variable  it is very hard for the compiler to put it in a register    As an example   void maybeModify int amp  x      may modify x in some way  void hurtTheCompilersOptimizer short size  int array             This function is designed to do something particularly troublesome        for optimizers  It will constantly call maybeModify on array 0  while        adding array 1  to array 2   array size-1   There s no real reason to        do this  other than to demonstrate the power of references      for  int i   2  i  lt   int size  i              maybeModify array 0            array i     array 1             An optimizing compiler may realize that we are accessing a 0  and a 1  quite a bunch  It would love to optimize the algorithm to   void hurtTheCompilersOptimizer short size  int array             Do the same thing as above  but instead of accessing array 1         all the time  access it once and store the result in a register         which is much faster to do arithmetic with      register int a0   a 0       register int a1   a 1      access a 1  once     for  int i   2  i  lt   int size  i              maybeModify a0      Give maybeModify a reference to a register         array i     a1      Use the saved register value over and over           a 0    a0     Store the modified a 0  back into the array     To make such an optimization  it needs to prove that nothing can change array 1  during the call  This is rather easy to do  i is never less than 2  so array i  can never refer to array 1   maybeModify   is given a0 as a reference  aliasing array 0    Because there is no  reference  arithmetic  the compiler just has to prove that maybeModify never gets the address of x  and it has proven that nothing changes array 1    It also has to prove that there are no ways a future call could read write a 0  while we have a temporary register copy of it in a0  This is often trivial to prove  because in many cases it is obvious that the reference is never stored in a permanent structure like a class instance   Now do the same thing with pointers  void maybeModify int  x      May modify x in some way  void hurtTheCompilersOptimizer short size  int array             Same operation  only now with pointers  making the        optimization trickier      for  int i   2  i  lt   int size  i              maybeModify  amp  array 0             array i     array 1             The behavior is the same  only now it is much harder to prove that maybeModify does not ever modify array 1   because we already gave it a pointer  the cat is out of the bag  Now it has to do the much more difficult proof  a static analysis of maybeModify to prove it never writes to  amp x   1  It also has to prove that it never saves off a pointer that can refer to array 0   which is just as tricky   Modern compilers are getting better and better at static analysis  but it is always nice to help them out and use references   Of course  barring such clever optimizations  compilers will indeed turn references into pointers when needed   EDIT  Five years after posting this answer  I found an actual technical difference where references are different than just a different way of looking at the same addressing concept   References can modify the lifespan of temporary objects in a way that pointers cannot   F createF int argument    void extending         const F amp  ref   createF 5       std  cout  lt  lt  ref getArgument    lt  lt  std  endl       Normally temporary objects such as the one created by the call to createF 5  are destroyed at the end of the expression   However  by binding that object to a reference  ref  C   will extend the lifespan of that temporary object until ref goes out of scope

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in simple words  we can say a reference is an alternative name for a variable whereas  a pointer is a variable that holds the address of another variable  e g   int a   20  int  amp r   a  r   40      now the value of a is changed to 40     int b  20  int  ptr  ptr    amp b     assigns address of b to ptr not the value

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Also  a reference that is a parameter to a function that is inlined may be handled differently than a pointer   void increment int  ptrint      ptrint       void increment int  amp refint    refint      void incptrtest         int testptr 0      increment  amp testptr     void increftest         int testref 0      increment testref       Many compilers when inlining the pointer version one will actually force a write to memory  we are taking the address explicitly    However  they will leave the reference in a register which is more optimal   Of course  for functions that are not inlined the pointer and reference generate the same code and it s always better to pass intrinsics by value than by reference if they are not modified and returned by the function

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Another interesting use of references is to supply a default argument of a user-defined type   class UDT   public     UDT     val d 33         UDT int val    val d val         virtual  UDT       private     int val d      class UDT Derived   public UDT   public     UDT Derived     UDT          virtual  UDT Derived           class Behavior   public     Behavior        const UDT  amp udt   UDT                 int main        Behavior b     take default     UDT u 88      Behavior c u       UDT Derived ud     Behavior d ud       return 1      The default flavor uses the  bind const reference to a temporary  aspect of references

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Another difference is that you can have pointers to a void type  and it means pointer to anything  but references to void are forbidden   int a  void   p    amp a     ok void  amp  p   a       forbidden   I can t say I m really happy with this particular difference  I would much prefer it would be allowed with the meaning reference to anything with an address and otherwise the same behavior for references  It would allow to define some equivalents of C library functions like memcpy using references

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I have an analogy for references and pointers  think of references as another name for an object and pointers as the address of an object      receives an alias of an int  an address of an int and an int value public void my function int amp  a int  b int c       int d   1     declares an integer named d     int  amp e   d     declares that e is an alias of d        using either d or e will yield the same result as d and e name the same object     int  f   e     invalid  you are trying to place an object in an address        imagine writting your name in an address field      int  g   f     writes an address to an address     g    amp d      amp d means get me the address of the object named d you could also        use  amp e as it is an alias of d and write it on g  which is an address so it s ok

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Maybe some metaphors will help   In the context of your desktop screenspace -      A reference requires you to specify an actual window  A pointer requires the location of a piece of space on screen that you assure it will contain zero or more instances of that window type

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If you want to be really pedantic  there is one thing you can do with a reference that you can t do with a pointer  extend the lifetime of a temporary object  In C   if you bind a const reference to a temporary object  the lifetime of that object becomes the lifetime of the reference   std  string s1    123   std  string s2    456    std  string s3 copy   s1   s2  const std  string amp  s3 reference   s1   s2    In this example s3 copy copies the temporary object that is a result of the concatenation  Whereas s3 reference in essence becomes the temporary object  It s really a reference to a temporary object that now has the same lifetime as the reference    If you try this without the const it should fail to compile  You cannot bind a non-const reference to a temporary object  nor can you take its address for that matter

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Some key pertinent details about references and pointers Pointers  Pointer variables are declared using the unary suffix declarator operator   Pointer objects are assigned an address value  for example  by assignment to an array object  the address of an object using the  amp  unary prefix operator  or assignment to the value of another pointer object A pointer can be reassigned any number of times  pointing to different objects A pointer is a variable that holds the assigned address   It takes up storage in memory equal to the size of the address for the target machine architecture A pointer can be mathematically manipulated  for instance  by the increment or addition operators   Hence  one can iterate with a pointer  etc  To get or set the contents of the object referred to by a pointer  one must use the unary prefix operator   to dereference it  References  References must be initialized when they are declared  References are declared using the unary suffix declarator operator  amp   When initializing a reference  one uses the name of the object to which they will refer directly  without the need for the unary prefix operator  amp  Once initialized  references cannot be pointed to something else by assignment or arithmetical manipulation There is no need to dereference the reference to get or set the contents of the object it refers to Assignment operations on the reference manipulate the contents of the object it points to  after initialization   not the reference itself  does not change where it points to  Arithmetic operations on the reference manipulate the contents of the object it points to  not the reference itself  does not change where it points to  In pretty much all implementations  the reference is actually stored as an address in memory of the referred to object   Hence  it takes up storage in memory equal to the size of the address for the target machine architecture just like a pointer object  Even though pointers and references are implemented in much the same way  quot under-the-hood  quot  the compiler treats them differently  resulting in all the differences described above  Article A recent article I wrote that goes into much greater detail than I can show here and should be very helpful for this question  especially about how things happen in memory  Arrays  Pointers and References Under the Hood In-Depth Article

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A reference to a pointer is possible in C    but the reverse is not possible means a pointer to a reference isn t possible  A reference to a pointer provides a cleaner syntax to modify the pointer  Look at this example    include lt iostream gt  using namespace std   void swap char    amp str1  char    amp str2      char  temp   str1    str1   str2    str2   temp     int main       char  str1    Hi     char  str2    Hello     swap str1  str2     cout lt  lt  str1 is   lt  lt str1 lt  lt endl    cout lt  lt  str2 is   lt  lt str2 lt  lt endl    return 0      And consider the C version of the above program  In C you have to use pointer to pointer  multiple indirection   and it leads to confusion and the program may look complicated    include lt stdio h gt     Swaps strings by swapping pointers    void swap1 char   str1 ptr  char   str2 ptr      char  temp    str1 ptr     str1 ptr    str2 ptr     str2 ptr   temp     int main       char  str1    Hi     char  str2    Hello     swap1  amp str1   amp str2     printf  str1 is  s  str2 is  s   str1  str2     return 0      Visit the following for more information about reference to pointer    C    Reference to Pointer Pointer-to-Pointer and Reference-to-Pointer   As I said  a pointer to a reference isn t possible  Try the following program    include  lt iostream gt  using namespace std   int main        int x   10     int  ptr    amp x     int  amp  ptr1   ptr

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You forgot the most important part   member-access with pointers uses - gt   member-access with references uses    foo bar is clearly superior to foo- gt bar in the same way that vi is clearly superior to Emacs  -

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At the risk of adding to confusion  I want to throw in some input  I m sure it mostly depends on how the compiler implements references  but in the case of gcc the idea that a reference can only point to a variable on the stack is not actually correct  take this for example    include  lt iostream gt  int main int argc  char   argv           Create a string on the heap     std  string  str ptr   new std  string  THIS IS A STRING           Dereference the string on the heap  and assign it to the reference     std  string  amp str ref    str ptr         Not even a compiler warning  At least with gcc        Now lets try to print it s value      std  cout  lt  lt  str ref  lt  lt  std  endl         It works  Now lets print and compare actual memory addresses     std  cout  lt  lt  str ptr  lt  lt         lt  lt   amp str ref  lt  lt  std  endl         Exactly the same  now remember to free the memory on the heap     delete str ptr      Which outputs this   THIS IS A STRING 0xbb2070   0xbb2070   If you notice even the memory addresses are exactly the same  meaning the reference is successfully pointing to a variable on the heap  Now if you really want to get freaky  this also works   int main int argc  char   argv           In the actual new declaration let immediately de-reference and assign it to the reference     std  string  amp str ref     new std  string  THIS IS A STRING            Once again  it works   at least in gcc      std  cout  lt  lt  str ref         Once again it prints fine  however we have no pointer to the heap allocation  right  So how do we free the space we just ignorantly created      delete  amp str ref        And  it works  because we are taking the memory address that the reference is     storing  and deleting it  which is all a pointer is doing  just we have to specify     the address with   amp   whereas a pointer does that implicitly  this is sort of like     calling delete  amp   str ptr    which also compiles and runs fine         Which outputs this   THIS IS A STRING   Therefore a reference IS a pointer under the hood  they both are just storing a memory address  where the address is pointing to is irrelevant  what do you think would happen if I called std  cout  lt  lt  str ref  AFTER calling delete  amp str ref  Well  obviously it compiles fine  but causes a segmentation fault at runtime because it s no longer pointing at a valid variable  we essentially have a broken reference that still exists  until it falls out of scope   but is useless   In other words  a reference is nothing but a pointer that has the pointer mechanics abstracted away  making it safer and easier to use  no accidental pointer math  no mixing up     and  -    etc    assuming you don t try any nonsense like my examples above     Now regardless of how a compiler handles references  it will always have some kind of pointer under the hood  because a reference must refer to a specific variable at a specific memory address for it to work as expected  there is no getting around this  hence the term  reference     The only major rule that s important to remember with references is that they must be defined at the time of declaration  with the exception of a reference in a header  in that case it must be defined in the constructor  after the object it s contained in is constructed it s too late to define it    Remember  my examples above are just that  examples demonstrating what a reference is  you would never want to use a reference in those ways  For proper usage of a reference there are plenty of answers on here already that hit the nail on the head

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What s a C   reference  for C programmers   A reference can be thought of as a constant pointer  not to be confused with a pointer to a constant value   with automatic indirection  ie the compiler will apply the   operator for you   All references must be initialized with a non-null value or compilation will fail  It s neither possible to get the address of a reference - the address operator will return the address of the referenced value instead - nor is it possible to do arithmetics on references   C programmers might dislike C   references as it will no longer be obvious when indirection happens or if an argument gets passed by value or by pointer without looking at function signatures   C   programmers might dislike using pointers as they are considered unsafe - although references aren t really any safer than constant pointers except in the most trivial cases - lack the convenience of automatic indirection and carry a different semantic connotation   Consider the following statement from the C   FAQ      Even though a reference is often implemented using an address in the   underlying assembly language  please do not think of a reference as a   funny looking pointer to an object  A reference is the object  It is   not a pointer to the object  nor a copy of the object  It is the   object    But if a reference really were the object  how could there be dangling references  In unmanaged languages  it s impossible for references to be any  safer  than pointers - there generally just isn t a way to reliably alias values across scope boundaries   Why I consider C   references useful  Coming from a C background  C   references may look like a somewhat silly concept  but one should still use them instead of pointers where possible  Automatic indirection is convenient  and references become especially useful when dealing with RAII - but not because of any perceived safety advantage  but rather because they make writing idiomatic code less awkward   RAII is one of the central concepts of C    but it interacts non-trivially with copying semantics  Passing objects by reference avoids these issues as no copying is involved  If references were not present in the language  you d have to use pointers instead  which are more cumbersome to use  thus violating the language design principle that the best-practice solution should be easier than the alternatives

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Difference between pointer and reference A pointer can be initialized to 0 and a reference not  In fact  a reference must also refer to an object  but a pointer can be the null pointer  int  p   0   But we can   t have int amp  p   0  and also int amp  p 5    In fact to do it properly  we must have declared and defined an object at the first then we can make a reference to that object  so the correct implementation of the previous code will be  Int x   0  Int y   5  Int amp  p   x  Int amp  p1   y   Another important point is that is we can make the declaration of the pointer without initialization however no such thing can be done in case of reference which must make a reference always to variable or object  However such use of a pointer is risky so generally we check if the pointer is actually is pointing to something or not  In case of a reference no such check is necessary  because we know already that referencing to an object during declaration is mandatory  Another difference is that pointer can point to another object however reference is always referencing to the same object  let   s take this example  Int a   6  b   5  Int amp  rf   a   Cout  lt  lt  rf  lt  lt  endl     The result we will get is 6  because rf is referencing to the value of a   rf   b  cout  lt  lt  a  lt  lt  endl     The result will be 5 because the value of b now will be stored into the address of a so the former value of a will be erased  Another point  When we have a template like an STL template such kind of a class template will always return a reference  not a pointer  to make easy reading or assigning new value using operator     Std   vector lt int gt v 10      Initialize a vector with 10 elements V 5    5     Writing the value 5 into the 6 element of our vector  so if the returned type of operator    was a pointer and not a reference we should write this  v 5  5  by making a reference we overwrite the element by using the assignment  quot   quot

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A reference can never be NULL

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Summary from answers and links below    A pointer can be re-assigned any number of times while a reference cannot be re-assigned after binding  Pointers can point nowhere  NULL   whereas a reference always refers to an object  You can t take the address of a reference like you can with pointers  There s no  reference arithmetic   but you can take the address of an object pointed by a reference and do pointer arithmetic on it as in  amp obj   5     To clarify a misconception      The C   standard is very careful to avoid dictating how a compiler may   implement references  but every C   compiler implements   references as pointers  That is  a declaration such as   int  amp ri   i        if it s not optimized away entirely  allocates the same amount of storage   as a pointer  and places the address   of i into that storage    So  a pointer and a reference both use the same amount of memory   As a general rule    Use references in function parameters and return types to provide useful and self-documenting interfaces  Use pointers for implementing algorithms and data structures    Interesting read    My all-time favorite C   FAQ lite  References vs  Pointers  An Introduction to References  References and const

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The difference is that non-constant pointer variable not to be confused with a pointer to constant  may be changed at some time during program execution  requires pointer semantics to be used  amp     operators  while references can be set upon initialization only that s why you can set them in constructor initializer list only  but not somehow else  and use ordinary value accessing semantics   Basically references were introduced to allow support for operators overloading as I had read in some very old book  As somebody stated in this thread - pointer can be set to 0 or whatever value you want  0 NULL  nullptr  means that the pointer is initialized with nothing  It is an error to dereference null pointer  But actually the pointer may contain a value that doesn t point to some correct memory location  References in their turn try not to allow a user to initialize a reference to something that cannot be referenced due to the fact that you always provide rvalue of correct type to it  Although there are a lot of ways to make reference variable be initialized to a wrong memory location - it is better for you not to dig this deep into details  On machine level both pointer and reference work uniformly - via pointers  Let s say in essential references are syntactic sugar  rvalue references are different to this - they are naturally stack heap objects

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There is one fundamental difference between pointers and references that I didn t see anyone had mentioned  references enable pass-by-reference semantics in function arguments  Pointers  although it is not visible at first do not  they only provide pass-by-value semantics  This has been very nicely described in this article   Regards   amp rzej

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Contrary to popular opinion  it is possible to have a reference that is NULL   int   p   NULL  int  amp  r    p  r   1      crash   if you re lucky    Granted  it is much harder to do with a reference - but if you manage it  you ll tear your hair out trying to find it  References are not inherently safe in C     Technically this is an invalid reference  not a null reference   C   doesn t support null references as a concept as you might find in other languages   There are other kinds of invalid references as well  Any invalid reference raises the spectre of undefined behavior  just as using an invalid pointer would   The actual error is in the dereferencing of the NULL pointer  prior to the assignment to a reference   But I m not aware of any compilers that will generate any errors on that condition - the error propagates to a point further along in the code  That s what makes this problem so insidious  Most of the time  if you dereference a NULL pointer  you crash right at that spot and it doesn t take much debugging to figure it out   My example above is short and contrived  Here s a more real-world example   class MyClass               virtual void DoSomething int int int int int       void Foo const MyClass  amp  bar                bar DoSomething i1 i2 i3 i4 i5       crash occurs here due to memory access violation - obvious why     MyClass   GetInstance         if  somecondition          return NULL             MyClass   p   GetInstance    Foo  p     I want to reiterate that the only way to get a null reference is through malformed code  and once you have it you re getting undefined behavior  It never makes sense to check for a null reference  for example you can try if  amp bar  NULL     but the compiler might optimize the statement out of existence  A valid reference can never be NULL so from the compiler s view the comparison is always false  and it is free to eliminate the if clause as dead code - this is the essence of undefined behavior   The proper way to stay out of trouble is to avoid dereferencing a NULL pointer to create a reference  Here s an automated way to accomplish this   template lt typename T gt  T amp  deref T  p        if  p    NULL          throw std  invalid argument std  string  NULL reference         return  p     MyClass   p   GetInstance    Foo deref p      For an older look at this problem from someone with better writing skills  see Null References from Jim Hyslop and Herb Sutter   For another example of the dangers of dereferencing a null pointer see Exposing undefined behavior when trying to port code to another platform by Raymond Chen

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There is a semantic difference that may appear esoteric if you are not familiar with studying computer languages in an abstract or even academic fashion   At the highest-level  the idea of references is that they are transparent  aliases   Your computer may use an address to make them work  but you re not supposed to worry about that  you re supposed to think of them as  just another name  for an existing object and the syntax reflects that  They are stricter than pointers so your compiler can more reliably warn you when you about to create a dangling reference  than when you are about to create a dangling pointer   Beyond that  there are of course some practical differences between pointers and references  The syntax to use them is obviously different  and you cannot  re-seat  references  have references to nothingness  or have pointers to references

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A reference is an alias for another variable whereas a pointer holds the memory address of a variable  References are generally used as function parameters so that the passed object is not the copy but the object itself        void fun int  amp a  int  amp b      A common usage of references      int a   0      int  amp b   a     b is an alias for a  Not so common to use

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I know references are syntactic sugar  so code is easier to read and write   This   A reference is not another way to implement a pointer  although it covers a huge pointer use case   A pointer is a datatype -- an address that in general points to a actual value   However it can be set to zero  or a couple of places after the address using address arithmetic  etc   A reference is  syntactic sugar  for a variable which has its own value   C only had pass by value semantics   Getting the address of the data a variable was referring to and sending that to a function was a way to pass by  reference    A reference shortcuts this semantically by  referring  to the original data location itself   So   int x   1  int  y    amp x  int  amp z   x    Y is an int pointer  pointing to the location where x is stored  X and Z refer to the same storage place  the stack or the heap    Alot of people have talked about the difference between the two  pointers and references  as if they are the same thing with different usages   They are not the same at all     1   A pointer can be re-assigned any number of times while a reference cannot be re-assigned after binding   -- a pointer is an address data type which points to data   A reference is another name for the data   So you can  reassign  a reference   You just can t reassign the data location it refers to   Just like you can t change the data location that  x  refers to  you can t do that to  z      x   2   y   2  z   2    The same   It is a reassignment    2   Pointers can point nowhere  NULL   whereas a reference always refers to an object  -- again with the confusion   The reference is just another name for the object   A null pointer means  semantically  that it isn t referring to anything  whereas the reference was created by saying it was another name for  x    Since  3   You can t take the address of a reference like you can with pointers  -- Yes you can   Again with the confusion   If you are trying to find the address of the pointer that is being used as a reference  that is a problem -- cause references are not pointers to the object   They are the object   So you can get the address of the object  and you can get the address of the pointer   Cause they are both getting the address of data  one the object s location in memory  the other the pointer to the objects location in memory    int  yz    amp z  -- legal int   yy    amp y  -- legal  int  yx    amp x  -- legal  notice how this looks like the z example   x and z are equivalent    4   There s no  reference arithmetic   -- again with the confusion -- since the example above has z being a reference to x and both are therefore integers   reference  arithmetic means for example adding 1 to the value referred to by x   x    z      y        what people assume is happening behind the scenes  but isn t  it would produce the same results in this example    y         this one adds to the pointer  and then dereferences it   It makes sense that a pointer datatype  an address  can be incremented   Just like an int can be incremented

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You can use the difference between references and pointers if you follow a convention for arguments passed to a function   Const references are for data passed into a function  and pointers are for data passed out of a function   In other languages  you can explicit notate this with keywords such as in and out  In C    you can declare  by convention  the equivalent  For example   void DoSomething const Foo amp  thisIsAnInput  Foo  thisIsAnOutput       if  thisIsAnOuput         thisIsAnOutput   thisIsAnInput      The use of references as inputs and pointers as outputs is part of the Google style guide

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I feel like there is yet another point that hasn t been covered here     Unlike the pointers  references are syntactically equivalent to the object they refer to  i e  any operation that can be applied to an object works for a reference  and with the exact same syntax  the exception is of course the initialization      While this may appear superficial  I believe this property is crucial for a number of C   features  for example      Templates  Since template parameters are duck-typed  syntactic properties of a type is all that matters  so often the same template can be used with both T and T amp    or std  reference wrapper lt T gt  which still relies on an implicit cast to T amp   Templates that cover both T amp  and T amp  amp  are even more common     Lvalues  Consider the statement str 0     X   Without references it would only work for c-strings  char  str   Returning the character by reference allows user-defined classes to have the same notation    Copy constructors  Syntactically it makes sense to pass objects to copy constructors  and not pointers to objects  But there is just no way for a copy constructor to take an object by value - it would result in a recursive call to the same copy constructor  This leaves references as the only option here    Operator overloads  With references it is possible to introduce indirection to an operator call - say  operator  const T amp  a  const T amp  b  while retaining the same infix notation  This also works for regular overloaded functions      These points empower a considerable part of C   and the standard library so this is quite a major property of references

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A pointer can be re-assigned  int x   5  int y   6  int  p  p    amp x  p    amp y   p   10  assert x    5   assert y    10    A reference cannot be re-bound  and must be bound at initialization  int x   5  int y   6  int  amp q     error int  amp r   x    A pointer variable has its own identity  a distinct  visible memory address that can be taken with the unary  amp  operator and a certain amount of space that can be measured with the sizeof operator  Using those operators on a reference returns a value corresponding to whatever the reference is bound to  the reference   s own address and size are invisible  Since the reference assumes the identity of the original variable in this way  it is convenient to think of a reference as another name for the same variable  int x   0  int  amp r   x  int  p    amp x  int  p2    amp r   assert p    p2       amp x     amp r assert  amp p     amp p2     You can have arbitrarily nested pointers to pointers offering extra levels of indirection  References only offer one level of indirection  int x   0  int y   0  int  p    amp x  int  q    amp y  int   pp    amp p     pp   2  pp    amp q      pp is now q   pp   4   assert y    4   assert x    2     A pointer can be assigned nullptr  whereas a reference must be bound to an existing object  If you try hard enough  you can bind a reference to nullptr  but this is undefined and will not behave consistently     the code below is undefined  your compiler may optimise it    differently  emit warnings  or outright refuse to compile it     int  amp r    static cast lt int   gt  nullptr       prints  quot null quot  under GCC 10 std  cout      lt  lt    amp r    nullptr            quot not null quot     quot null quot        lt  lt  std  endl   bool f int  amp r    return  amp r    nullptr        prints  quot not null quot  under GCC 10 std  cout      lt  lt   f  static cast lt int   gt  nullptr              quot not null quot     quot null quot        lt  lt  std  endl   You can  however  have a reference to a pointer whose value is nullptr   Pointers can iterate over an array  you can use    to go to the next item that a pointer is pointing to  and   4 to go to the 5th element   This is no matter what size the object is that the pointer points to   A pointer needs to be dereferenced with   to access the memory location it points to  whereas a reference can be used directly   A pointer to a class struct uses - gt  to access its members whereas a reference uses a     References cannot be put into an array  whereas pointers can be  Mentioned by user  litb   Const references can be bound to temporaries  Pointers cannot  not without some indirection   const int  amp x   int 12      legal C   int  y    amp int 12      illegal to take the address of a temporary   This makes const  amp  more convenient to use in argument lists and so forth

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There is a very important non-technical difference between pointers and references  An argument passed to a function by pointer is much more visible than an argument passed to a function by non-const reference  For example   void fn1 std  string s   void fn2 const std  string amp  s   void fn3 std  string amp  s   void fn4 std  string  s    void bar         std  string x      fn1 x       Cannot modify x     fn2 x       Cannot modify x  without const cast      fn3 x       CAN modify x      fn4  amp x      Can modify x  but is obvious about it      Back in C  a call that looks like fn x  can only be passed by value  so it definitely cannot modify x  to modify an argument you would need to pass a pointer fn  amp x   So if an argument wasn t preceded by an  amp  you knew it would not be modified   The converse   amp  means modified  was not true because you would sometimes have to pass large read-only structures by const pointer    Some argue that this is such a useful feature when reading code  that pointer parameters should always be used for modifiable parameters rather than non-const references  even if the function never expects a nullptr  That is  those people argue that function signatures like fn3   above should not be allowed  Google s C   style guidelines are an example of this

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Apart from syntactic sugar  a reference is a const pointer  not pointer to a const   You must establish what it refers to when you declare the reference variable  and you cannot change it later   Update  now that I think about it some more  there is an important difference   A const pointer s target can be replaced by taking its address and using a const cast   A reference s target cannot be replaced in any way short of UB   This should permit the compiler to do more optimization on a reference

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Beside all the answers here   you can implement operator overloading using references   my point operator  const my point amp  a  const my point amp  b      return   a x   b x  a y   b y        Using parameters as value would create temporary copies of the original arguments and using pointers would not invoke this function because of pointers arithmetics

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This program might help in comprehending the answer of the question  This is a simple program of a reference  j  and a pointer  ptr  pointing to variable  x     include lt iostream gt   using namespace std   int main     int  ptr 0  x 9     pointer and variable declaration ptr  amp x     pointer to variable  x  int  amp  j x     reference declaration  reference to variable  x   cout  lt  lt   x    lt  lt  x  lt  lt  endl   cout  lt  lt    amp x    lt  lt   amp x  lt  lt  endl   cout  lt  lt   j    lt  lt  j  lt  lt  endl   cout  lt  lt    amp j    lt  lt   amp j  lt  lt  endl   cout  lt  lt    ptr    lt  lt   ptr  lt  lt  endl   cout  lt  lt   ptr    lt  lt  ptr  lt  lt  endl   cout  lt  lt    amp ptr    lt  lt   amp ptr  lt  lt  endl      getch        Run the program and have a look at the output and you ll understand   Also  spare 10 minutes and watch this video  https   www youtube com watch v rlJrrGV0iOg

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This is based on the tutorial  What is written makes it more clear    gt  gt  gt  The address that locates a variable within memory is     what we call a reference to that variable   5th paragraph at page 63    gt  gt  gt  The variable that stores the reference to another     variable is what we call a pointer   3rd paragraph at page 64    Simply to remember that    gt  gt  gt  reference stands for memory location  gt  gt  gt  pointer is a reference container  Maybe because we will use it for several times  it is better to remember that reference     What s more  as we can refer to almost any pointer tutorial  a pointer is an object that is supported by pointer arithmetic which makes pointer similar to an array   Look at the following statement   int Tom 0   int  amp  alias Tom   Tom    alias Tom can be understood as an alias of a variable  different with typedef  which is alias of a type  Tom  It is also OK to forget the terminology of such statement is to create a reference of Tom

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Basic meaning of pointer    is  Value at address of  which means whatever address you provide it will give value at that address  Once you change the address it will give the new value  while reference variable used to reference any particular variable and which can t be change to reference any other variable in future

[c++] What are the differences between a pointer variable and a reference variable in C++?

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