Is it better in C to pass by value or pass by constant reference

Question

Is it better in C   to pass by value or pass by constant reference   I am wondering which is better practice   I realize that pass by constant reference should provide for better performance in the program because you are not making a copy of the variable

User · Accepted Answer

It used to be generally recommended best practice¹ to use pass by const ref for all types, except for builtin types (char, int, double, etc.), for iterators and for function objects (lambdas, classes deriving from std::*_function).

This was especially true before the existence of move semantics. The reason is simple: if you passed by value, a copy of the object had to be made and, except for very small objects, this is always more expensive than passing a reference.

With C++11, we have gained move semantics. In a nutshell, move semantics permit that, in some cases, an object can be passed “by value” without copying it. In particular, this is the case when the object that you are passing is an rvalue.

In itself, moving an object is still at least as expensive as passing by reference. However, in many cases a function will internally copy an object anyway — i.e. it will take ownership of the argument.²

In these situations we have the following (simplified) trade-off:

We can pass the object by reference, then copy internally.
We can pass the object by value.

“Pass by value” still causes the object to be copied, unless the object is an rvalue. In the case of an rvalue, the object can be moved instead, so that the second case is suddenly no longer “copy, then move” but “move, then (potentially) move again”.

For large objects that implement proper move constructors (such as vectors, strings …), the second case is then vastly more efficient than the first. Therefore, it is recommended to use pass by value if the function takes ownership of the argument, and if the object type supports efficient moving.

A historical note:

In fact, any modern compiler should be able to figure out when passing by value is expensive, and implicitly convert the call to use a const ref if possible.

In theory. In practice, compilers can’t always change this without breaking the function’s binary interface. In some special cases (when the function is inlined) the copy will actually be elided if the compiler can figure out that the original object won’t be changed through the actions in the function.

But in general the compiler can’t determine this, and the advent of move semantics in C++ has made this optimisation much less relevant.

¹ E.g. in Scott Meyers, Effective C++.

² This is especially often true for object constructors, which may take arguments and store them internally to be part of the constructed object’s state.

User · Answer

As it has been pointed out  it depends on the type   For built-in data types  it is best to pass by value   Even some very small structures  such as a pair of ints can perform better by passing by value   Here is an example  assume you have an integer value and you want pass it to another routine   If that value has been optimized to be stored in a register  then if you want to pass it be reference  it first must be stored in memory and then a pointer to that memory placed on the stack to perform the call   If it was being passed by value  all that is required is the register pushed onto the stack    The details are a bit more complicated than that given different calling systems and CPUs    If you are doing template programming  you are usually forced to always pass by const ref since you don t know the types being passed in   Passing penalties for passing something bad by value are much worse than the penalties of passing a built-in type by const ref

User · Answer

This is what i normally work by when designing the interface of a non-template function    Pass by value if the function does not want to modify the parameter and the value is cheap to copy  int  double  float  char  bool  etc    Notice that std  string  std  vector  and the rest of the containers in the standard library are NOT  Pass by const pointer if the value is expensive to copy and the function does not want to modify the value pointed to and NULL is a value that the function handles  Pass by non-const pointer if the value is expensive to copy and the function wants to modify the value pointed to and NULL is a value that the function handles  Pass by const reference when the value is expensive to copy and the function does not want to modify the value referred to and NULL would not be a valid value if a pointer was used instead  Pass by non-const reference when the value is expensive to copy and the function wants to modify the value referred to and NULL would not be a valid value if a pointer was used instead

User · Answer

As a rule passing by const reference is better  But if you need to modify you function argument locally you should better use passing by value  For some basic types the performance in general the same both for passing by value and by reference  Actually reference internally represented by pointer  that is why you can expect for instance that for pointer both passing are the same in terms of performance  or even passing by value can be faster because of needless dereference

User · Answer

As a rule of thumb  value for non-class types and const reference for classes  If a class is really small it s probably better to pass by value  but the difference is minimal  What you really want to avoid is passing some gigantic class by value and having it all duplicated - this will make a huge difference if you re passing  say  a std  vector with quite a few elements in it

User · Answer

As a rule passing by const reference is better  But if you need to modify you function argument locally you should better use passing by value  For some basic types the performance in general the same both for passing by value and by reference  Actually reference internally represented by pointer  that is why you can expect for instance that for pointer both passing are the same in terms of performance  or even passing by value can be faster because of needless dereference

User · Answer

As a rule passing by const reference is better  But if you need to modify you function argument locally you should better use passing by value  For some basic types the performance in general the same both for passing by value and by reference  Actually reference internally represented by pointer  that is why you can expect for instance that for pointer both passing are the same in terms of performance  or even passing by value can be faster because of needless dereference

User · Answer

Simple difference  - In function we have input and output parameter   so if your passing input and out parameter is same then use call by reference else if input and output parameter are different then better to use call by value    example  void amount int account   int deposit   int total    input parameter   account   deposit output paramteter  total   input and out is different use call by vaule   void amount int total   int deposit     input total deposit output total

User · Answer

As it has been pointed out  it depends on the type   For built-in data types  it is best to pass by value   Even some very small structures  such as a pair of ints can perform better by passing by value   Here is an example  assume you have an integer value and you want pass it to another routine   If that value has been optimized to be stored in a register  then if you want to pass it be reference  it first must be stored in memory and then a pointer to that memory placed on the stack to perform the call   If it was being passed by value  all that is required is the register pushed onto the stack    The details are a bit more complicated than that given different calling systems and CPUs    If you are doing template programming  you are usually forced to always pass by const ref since you don t know the types being passed in   Passing penalties for passing something bad by value are much worse than the penalties of passing a built-in type by const ref

User · Answer

Depends on the type   You are adding the small overhead of having to make a reference and dereference   For types with a size equal or smaller than pointers that are using the default copy ctor  it would probably be faster to pass by value

User · Answer

This is what i normally work by when designing the interface of a non-template function    Pass by value if the function does not want to modify the parameter and the value is cheap to copy  int  double  float  char  bool  etc    Notice that std  string  std  vector  and the rest of the containers in the standard library are NOT  Pass by const pointer if the value is expensive to copy and the function does not want to modify the value pointed to and NULL is a value that the function handles  Pass by non-const pointer if the value is expensive to copy and the function wants to modify the value pointed to and NULL is a value that the function handles  Pass by const reference when the value is expensive to copy and the function does not want to modify the value referred to and NULL would not be a valid value if a pointer was used instead  Pass by non-const reference when the value is expensive to copy and the function wants to modify the value referred to and NULL would not be a valid value if a pointer was used instead

User · Answer

Edit  New article by Dave Abrahams on cpp-next  Want speed  Pass by value     Pass by value for structs where the copying is cheap has the additional advantage that the compiler may assume that the objects don t alias  are not the same objects   Using pass-by-reference the compiler cannot assume that always  Simple example   foo   f   void bar foo g        g i   10      f- gt i   2      g i    5      the compiler can optimize it into  g i   15  f- gt i   2    since it knows that f and g doesn t share the same location  if g was a reference  foo  amp    the compiler couldn t have assumed that  since g i could then be aliased by f- i and have to have a value of 7  so the compiler would have to re-fetch the new value of g i from memory   For more pratical rules  here is a good set of rules found in Move Constructors article  highly recommended reading     If the function intends to change the argument as a side effect  take it by non-const reference   If the function doesn t modify its argument and the argument is of primitive type  take it by value  Otherwise take it by const reference  except in the following cases   If the function would then need to make a copy of the const reference anyway  take it by value      Primitive  above means basically small data types that are a few bytes long and aren t polymorphic  iterators  function objects  etc     or expensive to copy  In that paper  there is one other rule  The idea is that sometimes one wants to make a copy  in case the argument can t be modified   and sometimes one doesn t want  in case one wants to use the argument itself in the function if the argument was a temporary anyway  for example   The paper explains in detail how that can be done  In C  1x that technique can be used natively with language support  Until then  i would go with the above rules    Examples  To make a string uppercase and return the uppercase version  one should always pass by value  One has to take a copy of it anyway  one couldn t change the const reference directly  - so better make it as transparent as possible to the caller and make that copy early so that the caller can optimize as much as possible - as detailed in that paper   my  string uppercase my  string s       change s and return it        However  if you don t need to change the parameter anyway  take it by reference to const   bool all uppercase my  string const amp  s            check to see whether any character is uppercase        However  if you the purpose of the parameter is to write something into the argument  then pass it by non-const reference  bool try parse T text  my  string  amp out           try to parse  write result into out

User · Answer

Edit  New article by Dave Abrahams on cpp-next  Want speed  Pass by value     Pass by value for structs where the copying is cheap has the additional advantage that the compiler may assume that the objects don t alias  are not the same objects   Using pass-by-reference the compiler cannot assume that always  Simple example   foo   f   void bar foo g        g i   10      f- gt i   2      g i    5      the compiler can optimize it into  g i   15  f- gt i   2    since it knows that f and g doesn t share the same location  if g was a reference  foo  amp    the compiler couldn t have assumed that  since g i could then be aliased by f- i and have to have a value of 7  so the compiler would have to re-fetch the new value of g i from memory   For more pratical rules  here is a good set of rules found in Move Constructors article  highly recommended reading     If the function intends to change the argument as a side effect  take it by non-const reference   If the function doesn t modify its argument and the argument is of primitive type  take it by value  Otherwise take it by const reference  except in the following cases   If the function would then need to make a copy of the const reference anyway  take it by value      Primitive  above means basically small data types that are a few bytes long and aren t polymorphic  iterators  function objects  etc     or expensive to copy  In that paper  there is one other rule  The idea is that sometimes one wants to make a copy  in case the argument can t be modified   and sometimes one doesn t want  in case one wants to use the argument itself in the function if the argument was a temporary anyway  for example   The paper explains in detail how that can be done  In C  1x that technique can be used natively with language support  Until then  i would go with the above rules    Examples  To make a string uppercase and return the uppercase version  one should always pass by value  One has to take a copy of it anyway  one couldn t change the const reference directly  - so better make it as transparent as possible to the caller and make that copy early so that the caller can optimize as much as possible - as detailed in that paper   my  string uppercase my  string s       change s and return it        However  if you don t need to change the parameter anyway  take it by reference to const   bool all uppercase my  string const amp  s            check to see whether any character is uppercase        However  if you the purpose of the parameter is to write something into the argument  then pass it by non-const reference  bool try parse T text  my  string  amp out           try to parse  write result into out

User · Answer

Sounds like you got your answer   Passing by value is expensive  but gives you a copy to work with if you need it

User · Answer

As a rule passing by const reference is better  But if you need to modify you function argument locally you should better use passing by value  For some basic types the performance in general the same both for passing by value and by reference  Actually reference internally represented by pointer  that is why you can expect for instance that for pointer both passing are the same in terms of performance  or even passing by value can be faster because of needless dereference

User · Answer

As a rule of thumb  value for non-class types and const reference for classes  If a class is really small it s probably better to pass by value  but the difference is minimal  What you really want to avoid is passing some gigantic class by value and having it all duplicated - this will make a huge difference if you re passing  say  a std  vector with quite a few elements in it

User · Answer

Sounds like you got your answer   Passing by value is expensive  but gives you a copy to work with if you need it

User · Answer

Sounds like you got your answer   Passing by value is expensive  but gives you a copy to work with if you need it

User · Answer

Simple difference  - In function we have input and output parameter   so if your passing input and out parameter is same then use call by reference else if input and output parameter are different then better to use call by value    example  void amount int account   int deposit   int total    input parameter   account   deposit output paramteter  total   input and out is different use call by vaule   void amount int total   int deposit     input total deposit output total

User · Answer

As it has been pointed out  it depends on the type   For built-in data types  it is best to pass by value   Even some very small structures  such as a pair of ints can perform better by passing by value   Here is an example  assume you have an integer value and you want pass it to another routine   If that value has been optimized to be stored in a register  then if you want to pass it be reference  it first must be stored in memory and then a pointer to that memory placed on the stack to perform the call   If it was being passed by value  all that is required is the register pushed onto the stack    The details are a bit more complicated than that given different calling systems and CPUs    If you are doing template programming  you are usually forced to always pass by const ref since you don t know the types being passed in   Passing penalties for passing something bad by value are much worse than the penalties of passing a built-in type by const ref

User · Answer

Depends on the type   You are adding the small overhead of having to make a reference and dereference   For types with a size equal or smaller than pointers that are using the default copy ctor  it would probably be faster to pass by value

User · Answer

Sounds like you got your answer   Passing by value is expensive  but gives you a copy to work with if you need it

User · Answer

As it has been pointed out  it depends on the type   For built-in data types  it is best to pass by value   Even some very small structures  such as a pair of ints can perform better by passing by value   Here is an example  assume you have an integer value and you want pass it to another routine   If that value has been optimized to be stored in a register  then if you want to pass it be reference  it first must be stored in memory and then a pointer to that memory placed on the stack to perform the call   If it was being passed by value  all that is required is the register pushed onto the stack    The details are a bit more complicated than that given different calling systems and CPUs    If you are doing template programming  you are usually forced to always pass by const ref since you don t know the types being passed in   Passing penalties for passing something bad by value are much worse than the penalties of passing a built-in type by const ref

User · Answer

As a rule of thumb  value for non-class types and const reference for classes  If a class is really small it s probably better to pass by value  but the difference is minimal  What you really want to avoid is passing some gigantic class by value and having it all duplicated - this will make a huge difference if you re passing  say  a std  vector with quite a few elements in it

User · Answer

Pass by value for small types   Pass by const references for big types  the definition of big can vary between machines  BUT  in C  11  pass by value if you are going to consume the data  since you can exploit move semantics  For example   class Person    public    Person std  string name    name  std  move name       private    std  string name        Now the calling code would do   Person p std  string  Albert       And only one object would be created and moved directly into member name  in class Person  If you pass by const reference  a copy will have to be made for putting it into name

User · Answer

Pass by value for small types   Pass by const references for big types  the definition of big can vary between machines  BUT  in C  11  pass by value if you are going to consume the data  since you can exploit move semantics  For example   class Person    public    Person std  string name    name  std  move name       private    std  string name        Now the calling code would do   Person p std  string  Albert       And only one object would be created and moved directly into member name  in class Person  If you pass by const reference  a copy will have to be made for putting it into name

User · Answer

As a rule of thumb  value for non-class types and const reference for classes  If a class is really small it s probably better to pass by value  but the difference is minimal  What you really want to avoid is passing some gigantic class by value and having it all duplicated - this will make a huge difference if you re passing  say  a std  vector with quite a few elements in it

User · Answer

Edit  New article by Dave Abrahams on cpp-next  Want speed  Pass by value     Pass by value for structs where the copying is cheap has the additional advantage that the compiler may assume that the objects don t alias  are not the same objects   Using pass-by-reference the compiler cannot assume that always  Simple example   foo   f   void bar foo g        g i   10      f- gt i   2      g i    5      the compiler can optimize it into  g i   15  f- gt i   2    since it knows that f and g doesn t share the same location  if g was a reference  foo  amp    the compiler couldn t have assumed that  since g i could then be aliased by f- i and have to have a value of 7  so the compiler would have to re-fetch the new value of g i from memory   For more pratical rules  here is a good set of rules found in Move Constructors article  highly recommended reading     If the function intends to change the argument as a side effect  take it by non-const reference   If the function doesn t modify its argument and the argument is of primitive type  take it by value  Otherwise take it by const reference  except in the following cases   If the function would then need to make a copy of the const reference anyway  take it by value      Primitive  above means basically small data types that are a few bytes long and aren t polymorphic  iterators  function objects  etc     or expensive to copy  In that paper  there is one other rule  The idea is that sometimes one wants to make a copy  in case the argument can t be modified   and sometimes one doesn t want  in case one wants to use the argument itself in the function if the argument was a temporary anyway  for example   The paper explains in detail how that can be done  In C  1x that technique can be used natively with language support  Until then  i would go with the above rules    Examples  To make a string uppercase and return the uppercase version  one should always pass by value  One has to take a copy of it anyway  one couldn t change the const reference directly  - so better make it as transparent as possible to the caller and make that copy early so that the caller can optimize as much as possible - as detailed in that paper   my  string uppercase my  string s       change s and return it        However  if you don t need to change the parameter anyway  take it by reference to const   bool all uppercase my  string const amp  s            check to see whether any character is uppercase        However  if you the purpose of the parameter is to write something into the argument  then pass it by non-const reference  bool try parse T text  my  string  amp out           try to parse  write result into out

User · Answer

Depends on the type   You are adding the small overhead of having to make a reference and dereference   For types with a size equal or smaller than pointers that are using the default copy ctor  it would probably be faster to pass by value

User · Answer

Depends on the type   You are adding the small overhead of having to make a reference and dereference   For types with a size equal or smaller than pointers that are using the default copy ctor  it would probably be faster to pass by value

User · Answer

Edit  New article by Dave Abrahams on cpp-next  Want speed  Pass by value     Pass by value for structs where the copying is cheap has the additional advantage that the compiler may assume that the objects don t alias  are not the same objects   Using pass-by-reference the compiler cannot assume that always  Simple example   foo   f   void bar foo g        g i   10      f- gt i   2      g i    5      the compiler can optimize it into  g i   15  f- gt i   2    since it knows that f and g doesn t share the same location  if g was a reference  foo  amp    the compiler couldn t have assumed that  since g i could then be aliased by f- i and have to have a value of 7  so the compiler would have to re-fetch the new value of g i from memory   For more pratical rules  here is a good set of rules found in Move Constructors article  highly recommended reading     If the function intends to change the argument as a side effect  take it by non-const reference   If the function doesn t modify its argument and the argument is of primitive type  take it by value  Otherwise take it by const reference  except in the following cases   If the function would then need to make a copy of the const reference anyway  take it by value      Primitive  above means basically small data types that are a few bytes long and aren t polymorphic  iterators  function objects  etc     or expensive to copy  In that paper  there is one other rule  The idea is that sometimes one wants to make a copy  in case the argument can t be modified   and sometimes one doesn t want  in case one wants to use the argument itself in the function if the argument was a temporary anyway  for example   The paper explains in detail how that can be done  In C  1x that technique can be used natively with language support  Until then  i would go with the above rules    Examples  To make a string uppercase and return the uppercase version  one should always pass by value  One has to take a copy of it anyway  one couldn t change the const reference directly  - so better make it as transparent as possible to the caller and make that copy early so that the caller can optimize as much as possible - as detailed in that paper   my  string uppercase my  string s       change s and return it        However  if you don t need to change the parameter anyway  take it by reference to const   bool all uppercase my  string const amp  s            check to see whether any character is uppercase        However  if you the purpose of the parameter is to write something into the argument  then pass it by non-const reference  bool try parse T text  my  string  amp out           try to parse  write result into out

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