Pass by Reference Value in C

Question

I would like to clarify the differences between by value and by reference   I drew a picture     So  for passing by value   a copy of an identical object is created with a different reference  and the local variable is assigned the new reference  so to point to the new copy  How to understand the words    If the function modifies that value  the modifications appear also within the scope of the calling function for both passing by value and by reference    Thanks

User · Answer

I m not sure if I understand your question correctly   It is a bit unclear   However  what might be confusing you is the following    When passing by reference  a reference to the same object is passed to the function being called   Any changes to the object will be reflected in the original object and hence the caller will see it  When passing by value  the copy constructor will be called   The default copy constructor will only do a shallow copy  hence  if the called function modifies an integer in the object  this will not be seen by the calling function  but if the function changes a data structure pointed to by a pointer within the object  then this will be seen by the caller due to the shallow copy    I might have mis-understood your question  but I thought I would give it a stab anyway

User · Answer

I think much confusion is generated by not communicating what is meant by passed by reference  When some people say pass by reference they usually mean not the argument itself  but rather the object being referenced  Some other say that pass by reference means that the object can t be changed in the callee  Example   struct Object       int i      void sample Object  o       1     o- gt i       void sample Object const amp  o       2        nothing useful here       void sample Object  amp  o       3     o i       void sample1 Object o       4     o i       int main         Object obj     10        Object const obj c     10         sample  amp obj      calls 1     sample obj     calls 3     sample obj c      calls 2     sample1 obj      calls 4     Some people would claim that 1 and 3 are pass by reference  while 2 would be pass by value  Another group of people say all but the last is pass by reference  because the object itself is not copied    I would like to draw a definition of that here what i claim to be pass by reference  A general overview over it can be found here  Difference between pass by reference and pass by value  The first and last are pass by value  and the middle two are pass by reference       sample  amp obj             yields a  Object    Passes a  pointer  to the object by value             The caller can change the pointer  the parameter   but that            won t change the temporary pointer created on the call side  the argument         sample obj            passes the object by  reference   It denotes the object itself  The callee           has got a reference parameter       sample obj c             also passes  by reference   the reference parameter references the           same object like the argument expression        sample1 obj             pass by value  The parameter object denotes a different object than the            one passed in    I vote for the following definition      An argument  1 3 1  is passed by reference if and only if the corresponding parameter of the function that s called has reference type and the reference parameter binds directly to the argument expression  8 5 3 4   In all other cases  we have to do with pass by value    That means that the following is pass by value   void f1 Object const amp  o   f1 Object        1  void f2 int const amp  i   f2 42      2  void f3 Object o   f3 Object            3 Object o1  f3 o1      4  void f4 Object  o   Object o1  f4  amp o1      5   1 is pass by value  because it s not directly bound  The implementation may copy the temporary and then bind that temporary to the reference  2 is pass by value  because the implementation initializes a temporary of the literal and then binds to the reference  3 is pass by value  because the parameter has not reference type  4 is pass by value for the same reason  5 is pass by value because the parameter has not got reference type  The following cases are pass by reference  by the rules of 8 5 3 4 and others    void f1 Object   amp  op   Object a  Object  op1    amp a  f1 op1      1  void f2 Object const amp  op   Object b  f2 b      2  struct A      struct B   operator A amp      static A a  return a       void f3 A  amp    B b  f3 b      passes the static a by reference

User · Answer

I m not sure if I understand your question correctly   It is a bit unclear   However  what might be confusing you is the following    When passing by reference  a reference to the same object is passed to the function being called   Any changes to the object will be reflected in the original object and hence the caller will see it  When passing by value  the copy constructor will be called   The default copy constructor will only do a shallow copy  hence  if the called function modifies an integer in the object  this will not be seen by the calling function  but if the function changes a data structure pointed to by a pointer within the object  then this will be seen by the caller due to the shallow copy    I might have mis-understood your question  but I thought I would give it a stab anyway

User · Answer

I think much confusion is generated by not communicating what is meant by passed by reference  When some people say pass by reference they usually mean not the argument itself  but rather the object being referenced  Some other say that pass by reference means that the object can t be changed in the callee  Example   struct Object       int i      void sample Object  o       1     o- gt i       void sample Object const amp  o       2        nothing useful here       void sample Object  amp  o       3     o i       void sample1 Object o       4     o i       int main         Object obj     10        Object const obj c     10         sample  amp obj      calls 1     sample obj     calls 3     sample obj c      calls 2     sample1 obj      calls 4     Some people would claim that 1 and 3 are pass by reference  while 2 would be pass by value  Another group of people say all but the last is pass by reference  because the object itself is not copied    I would like to draw a definition of that here what i claim to be pass by reference  A general overview over it can be found here  Difference between pass by reference and pass by value  The first and last are pass by value  and the middle two are pass by reference       sample  amp obj             yields a  Object    Passes a  pointer  to the object by value             The caller can change the pointer  the parameter   but that            won t change the temporary pointer created on the call side  the argument         sample obj            passes the object by  reference   It denotes the object itself  The callee           has got a reference parameter       sample obj c             also passes  by reference   the reference parameter references the           same object like the argument expression        sample1 obj             pass by value  The parameter object denotes a different object than the            one passed in    I vote for the following definition      An argument  1 3 1  is passed by reference if and only if the corresponding parameter of the function that s called has reference type and the reference parameter binds directly to the argument expression  8 5 3 4   In all other cases  we have to do with pass by value    That means that the following is pass by value   void f1 Object const amp  o   f1 Object        1  void f2 int const amp  i   f2 42      2  void f3 Object o   f3 Object            3 Object o1  f3 o1      4  void f4 Object  o   Object o1  f4  amp o1      5   1 is pass by value  because it s not directly bound  The implementation may copy the temporary and then bind that temporary to the reference  2 is pass by value  because the implementation initializes a temporary of the literal and then binds to the reference  3 is pass by value  because the parameter has not reference type  4 is pass by value for the same reason  5 is pass by value because the parameter has not got reference type  The following cases are pass by reference  by the rules of 8 5 3 4 and others    void f1 Object   amp  op   Object a  Object  op1    amp a  f1 op1      1  void f2 Object const amp  op   Object b  f2 b      2  struct A      struct B   operator A amp      static A a  return a       void f3 A  amp    B b  f3 b      passes the static a by reference

User · Answer

I m not sure if I understand your question correctly   It is a bit unclear   However  what might be confusing you is the following    When passing by reference  a reference to the same object is passed to the function being called   Any changes to the object will be reflected in the original object and hence the caller will see it  When passing by value  the copy constructor will be called   The default copy constructor will only do a shallow copy  hence  if the called function modifies an integer in the object  this will not be seen by the calling function  but if the function changes a data structure pointed to by a pointer within the object  then this will be seen by the caller due to the shallow copy    I might have mis-understood your question  but I thought I would give it a stab anyway

User · Answer

I think much confusion is generated by not communicating what is meant by passed by reference  When some people say pass by reference they usually mean not the argument itself  but rather the object being referenced  Some other say that pass by reference means that the object can t be changed in the callee  Example   struct Object       int i      void sample Object  o       1     o- gt i       void sample Object const amp  o       2        nothing useful here       void sample Object  amp  o       3     o i       void sample1 Object o       4     o i       int main         Object obj     10        Object const obj c     10         sample  amp obj      calls 1     sample obj     calls 3     sample obj c      calls 2     sample1 obj      calls 4     Some people would claim that 1 and 3 are pass by reference  while 2 would be pass by value  Another group of people say all but the last is pass by reference  because the object itself is not copied    I would like to draw a definition of that here what i claim to be pass by reference  A general overview over it can be found here  Difference between pass by reference and pass by value  The first and last are pass by value  and the middle two are pass by reference       sample  amp obj             yields a  Object    Passes a  pointer  to the object by value             The caller can change the pointer  the parameter   but that            won t change the temporary pointer created on the call side  the argument         sample obj            passes the object by  reference   It denotes the object itself  The callee           has got a reference parameter       sample obj c             also passes  by reference   the reference parameter references the           same object like the argument expression        sample1 obj             pass by value  The parameter object denotes a different object than the            one passed in    I vote for the following definition      An argument  1 3 1  is passed by reference if and only if the corresponding parameter of the function that s called has reference type and the reference parameter binds directly to the argument expression  8 5 3 4   In all other cases  we have to do with pass by value    That means that the following is pass by value   void f1 Object const amp  o   f1 Object        1  void f2 int const amp  i   f2 42      2  void f3 Object o   f3 Object            3 Object o1  f3 o1      4  void f4 Object  o   Object o1  f4  amp o1      5   1 is pass by value  because it s not directly bound  The implementation may copy the temporary and then bind that temporary to the reference  2 is pass by value  because the implementation initializes a temporary of the literal and then binds to the reference  3 is pass by value  because the parameter has not reference type  4 is pass by value for the same reason  5 is pass by value because the parameter has not got reference type  The following cases are pass by reference  by the rules of 8 5 3 4 and others    void f1 Object   amp  op   Object a  Object  op1    amp a  f1 op1      1  void f2 Object const amp  op   Object b  f2 b      2  struct A      struct B   operator A amp      static A a  return a       void f3 A  amp    B b  f3 b      passes the static a by reference

User · Answer

My understanding of the words  If the function modifies that value  the modifications appear also within the scope of the calling function for both passing by value and by reference  is that they are an error   Modifications made in a called function are not in scope of the calling function when passing by value   Either you have mistyped the quoted words or they have been extracted out of whatever context made what appears to be wrong  right   Could you please ensure you have correctly quoted your source and if there are no errors there give more of the text surrounding that statement in the source material

User · Answer

When passing by value   void func Object o     and then calling   func a     you will construct an Object on the stack  and within the implementation of func it will be referenced by o  This might still be a shallow copy  the internals of a and o might point to the same data   so a might be changed  However if o is a deep copy of a  then a will not change   When passing by reference   void func2 Object amp  o     and then calling  func2 a     you will only be giving a new way to reference a   a  and  o  are two names for the same object  Changing o inside func2 will make those changes visible to the caller  who knows the object by the name  a

User · Answer

My understanding of the words  If the function modifies that value  the modifications appear also within the scope of the calling function for both passing by value and by reference  is that they are an error   Modifications made in a called function are not in scope of the calling function when passing by value   Either you have mistyped the quoted words or they have been extracted out of whatever context made what appears to be wrong  right   Could you please ensure you have correctly quoted your source and if there are no errors there give more of the text surrounding that statement in the source material

User · Answer

My understanding of the words  If the function modifies that value  the modifications appear also within the scope of the calling function for both passing by value and by reference  is that they are an error   Modifications made in a called function are not in scope of the calling function when passing by value   Either you have mistyped the quoted words or they have been extracted out of whatever context made what appears to be wrong  right   Could you please ensure you have correctly quoted your source and if there are no errors there give more of the text surrounding that statement in the source material

User · Answer

Thanks so much everyone for all these input   I quoted that sentence from a lecture note online  http   www cs cornell edu courses cs213 2002fa lectures Lecture02 Lecture02 pdf  the first page the 6th slide    Pass by VALUE The value of a variable is passed along to the function If the function modifies that value  the modifications stay within the scope of that function   Pass by REFERENCE A reference to the variable is passed along to the function If the function modifies that value  the modifications appear also within the scope of the calling function      Thanks so much again

User · Answer

As I parse it  those words are wrong   It should read  If the function modifies that value  the modifications appear also within the scope of the calling function when passing by reference  but not when passing by value

User · Answer

Thanks so much everyone for all these input   I quoted that sentence from a lecture note online  http   www cs cornell edu courses cs213 2002fa lectures Lecture02 Lecture02 pdf  the first page the 6th slide    Pass by VALUE The value of a variable is passed along to the function If the function modifies that value  the modifications stay within the scope of that function   Pass by REFERENCE A reference to the variable is passed along to the function If the function modifies that value  the modifications appear also within the scope of the calling function      Thanks so much again

User · Answer

My understanding of the words  If the function modifies that value  the modifications appear also within the scope of the calling function for both passing by value and by reference  is that they are an error   Modifications made in a called function are not in scope of the calling function when passing by value   Either you have mistyped the quoted words or they have been extracted out of whatever context made what appears to be wrong  right   Could you please ensure you have correctly quoted your source and if there are no errors there give more of the text surrounding that statement in the source material

User · Answer

When passing by value   void func Object o     and then calling   func a     you will construct an Object on the stack  and within the implementation of func it will be referenced by o  This might still be a shallow copy  the internals of a and o might point to the same data   so a might be changed  However if o is a deep copy of a  then a will not change   When passing by reference   void func2 Object amp  o     and then calling  func2 a     you will only be giving a new way to reference a   a  and  o  are two names for the same object  Changing o inside func2 will make those changes visible to the caller  who knows the object by the name  a

User · Answer

When passing by value   void func Object o     and then calling   func a     you will construct an Object on the stack  and within the implementation of func it will be referenced by o  This might still be a shallow copy  the internals of a and o might point to the same data   so a might be changed  However if o is a deep copy of a  then a will not change   When passing by reference   void func2 Object amp  o     and then calling  func2 a     you will only be giving a new way to reference a   a  and  o  are two names for the same object  Changing o inside func2 will make those changes visible to the caller  who knows the object by the name  a

User · Answer

I think much confusion is generated by not communicating what is meant by passed by reference  When some people say pass by reference they usually mean not the argument itself  but rather the object being referenced  Some other say that pass by reference means that the object can t be changed in the callee  Example   struct Object       int i      void sample Object  o       1     o- gt i       void sample Object const amp  o       2        nothing useful here       void sample Object  amp  o       3     o i       void sample1 Object o       4     o i       int main         Object obj     10        Object const obj c     10         sample  amp obj      calls 1     sample obj     calls 3     sample obj c      calls 2     sample1 obj      calls 4     Some people would claim that 1 and 3 are pass by reference  while 2 would be pass by value  Another group of people say all but the last is pass by reference  because the object itself is not copied    I would like to draw a definition of that here what i claim to be pass by reference  A general overview over it can be found here  Difference between pass by reference and pass by value  The first and last are pass by value  and the middle two are pass by reference       sample  amp obj             yields a  Object    Passes a  pointer  to the object by value             The caller can change the pointer  the parameter   but that            won t change the temporary pointer created on the call side  the argument         sample obj            passes the object by  reference   It denotes the object itself  The callee           has got a reference parameter       sample obj c             also passes  by reference   the reference parameter references the           same object like the argument expression        sample1 obj             pass by value  The parameter object denotes a different object than the            one passed in    I vote for the following definition      An argument  1 3 1  is passed by reference if and only if the corresponding parameter of the function that s called has reference type and the reference parameter binds directly to the argument expression  8 5 3 4   In all other cases  we have to do with pass by value    That means that the following is pass by value   void f1 Object const amp  o   f1 Object        1  void f2 int const amp  i   f2 42      2  void f3 Object o   f3 Object            3 Object o1  f3 o1      4  void f4 Object  o   Object o1  f4  amp o1      5   1 is pass by value  because it s not directly bound  The implementation may copy the temporary and then bind that temporary to the reference  2 is pass by value  because the implementation initializes a temporary of the literal and then binds to the reference  3 is pass by value  because the parameter has not reference type  4 is pass by value for the same reason  5 is pass by value because the parameter has not got reference type  The following cases are pass by reference  by the rules of 8 5 3 4 and others    void f1 Object   amp  op   Object a  Object  op1    amp a  f1 op1      1  void f2 Object const amp  op   Object b  f2 b      2  struct A      struct B   operator A amp      static A a  return a       void f3 A  amp    B b  f3 b      passes the static a by reference

User · Answer

Thanks so much everyone for all these input   I quoted that sentence from a lecture note online  http   www cs cornell edu courses cs213 2002fa lectures Lecture02 Lecture02 pdf  the first page the 6th slide    Pass by VALUE The value of a variable is passed along to the function If the function modifies that value  the modifications stay within the scope of that function   Pass by REFERENCE A reference to the variable is passed along to the function If the function modifies that value  the modifications appear also within the scope of the calling function      Thanks so much again

User · Answer

As I parse it  those words are wrong   It should read  If the function modifies that value  the modifications appear also within the scope of the calling function when passing by reference  but not when passing by value

User · Answer

As I parse it  those words are wrong   It should read  If the function modifies that value  the modifications appear also within the scope of the calling function when passing by reference  but not when passing by value

User · Answer

When passing by value   void func Object o     and then calling   func a     you will construct an Object on the stack  and within the implementation of func it will be referenced by o  This might still be a shallow copy  the internals of a and o might point to the same data   so a might be changed  However if o is a deep copy of a  then a will not change   When passing by reference   void func2 Object amp  o     and then calling  func2 a     you will only be giving a new way to reference a   a  and  o  are two names for the same object  Changing o inside func2 will make those changes visible to the caller  who knows the object by the name  a

User · Answer

Thanks so much everyone for all these input   I quoted that sentence from a lecture note online  http   www cs cornell edu courses cs213 2002fa lectures Lecture02 Lecture02 pdf  the first page the 6th slide    Pass by VALUE The value of a variable is passed along to the function If the function modifies that value  the modifications stay within the scope of that function   Pass by REFERENCE A reference to the variable is passed along to the function If the function modifies that value  the modifications appear also within the scope of the calling function      Thanks so much again

User · Answer

I m not sure if I understand your question correctly   It is a bit unclear   However  what might be confusing you is the following    When passing by reference  a reference to the same object is passed to the function being called   Any changes to the object will be reflected in the original object and hence the caller will see it  When passing by value  the copy constructor will be called   The default copy constructor will only do a shallow copy  hence  if the called function modifies an integer in the object  this will not be seen by the calling function  but if the function changes a data structure pointed to by a pointer within the object  then this will be seen by the caller due to the shallow copy    I might have mis-understood your question  but I thought I would give it a stab anyway

User · Answer

As I parse it  those words are wrong   It should read  If the function modifies that value  the modifications appear also within the scope of the calling function when passing by reference  but not when passing by value

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