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 = "") //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:

     root:~$ g++ copy_elision.cpp -fno-elide-constructors
     root:~$ ./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

 

　　Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
　　　　

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　　2013-11-26　　11:13:29