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.

 1 #include <iostream>

 2 using namespace std;

 3

 4 class B

 5 {

 6 public:

 7     B(const char* str = "\0") //default constructor

 8     {

 9         cout << "Constructor called" << endl;

10     }

11

12     B(const B &b)  //copy constructor

13     {

14         cout << "Copy constructor called" << endl;

15     }

16 };

17

18 int main()

19 {

20     B ob = "copy me";

21     return 0;

22 }

　　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

巴特西

Copy elision in C++

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