Passing std string by Value or Reference

Question

Possible Duplicate    Are the days of passing const std  string  amp  as a parameter over        Should I pass std  string by value or by reference  to a un-inlined function  if move semantics is supported  And what about implementations using small string optimization  SSO

User · Answer

I believe the normal answer is that it should be passed by value if you need to make a copy of it in your function. Pass it by const reference otherwise.

Here is a good discussion: http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/

User · Answer

Check this answer for C  11   Basically  if you pass an lvalue the rvalue reference   From this article   void f1 String s        vector lt String gt  v      v push back std  move s      void f2 const String  amp s        vector lt String gt  v      v push back s        For lvalue argument     f1    has one extra copy to pass the argument because it is by-value  while    f2    has one extra copy to call push back  So no difference  for rvalue argument  the compiler has to create a temporary    String L           and pass the temporary to    f1    or    f2    anyway  Because    f2    can take advantage of move ctor when the argument is a temporary  which is an rvalue   the costs to pass the argument are the same now for    f1    and    f2       Continuing    This means in C  11 we can get better performance by using pass-by-value approach when    The parameter type supports move semantics - All standard library components do in C  11 The cost of move constructor is much cheaper than the copy constructor  both the time and stack usage   Inside the function  the parameter type will be passed to another function or operation which supports both copy and move  It is common to pass a temporary as the argument - You can organize you code to do this more       OTOH  for C  98 it is best to pass by reference - less data gets copied around   Passing const or non const depend of whether you need to change the argument or not

User · Answer

There are multiple answers based on what you are doing with the string   1  Using the string as an id  will not be modified   Passing it in by const reference is probably the best idea here   std  string const amp    2  Modifying the string but not wanting the caller to see that change   Passing it in by value is preferable   std  string   3  Modifying the string but wanting the caller to see that change   Passing it in by reference is preferable   std  string  amp    4  Sending the string into the function and the caller of the function will never use the string again   Using move semantics might be an option  std  string  amp  amp

[c++] Passing std::string by Value or Reference

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