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
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2013-11-26 11:13:29