Copy elision in C++

Copy elision (or Copy omission) is a compiler optimization technique that avoids unnecessary copying of objects. Now a days, almost every compiler uses it. Let us understand it with the help of an example.

#include <iostream>
using namespace std;
 
class B
{
public:    
    B(const char* str = "\0") //default constructor
    {
        cout << "Constructor called" << endl;
    }    
    
    B(const B &b)  //copy constructor
    {
        cout << "Copy constructor called" << endl;
    } 
};
 
int main()
{ 
    B ob = "copy me"; 
    return 0;
}

The output of above program is:

Constructor called

Why copy constructor is not called?
According to theory, when the object “ob” is being constructed, one argument constructor is used to convert “copy me” to a temporary object & that temporary object is copied to the object “ob”. So the statement

     B ob = "copy me"; 

should be broken down by the compiler as

     B ob = B("copy me");

However, most of the C++ compilers avoid such overheads of creating a temporary object & then copying it.

The modern compilers break down the statement
    B ob = "copy me"; //copy initialization
as
    B ob("copy me"); //direct initialization
and thus eliding call to copy constructor.

However, if we still want to ensure that the compiler doesn’t elide the call to copy constructor [disable the copy elision], we can compile the program using “-fno-elide-constructors” option with g++ and see the output as following:

  aashish@aashish-ThinkPad-SL400:~$ g++ copy_elision.cpp -fno-elide-constructors
  aashish@aashish-ThinkPad-SL400:~$ ./a.out
  Constructor called
  Copy constructor called

If “-fno-elide-constructors” option is used, first default constructor is called to create a temporary object, then copy constructor is called to copy the temporary object to ob.

Reference:
http://en.wikipedia.org/wiki/Copy_elision

This article is compiled by Aashish Barnwal and reviewed by GeeksforGeeks team. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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