Copy Constructor is not invoked [duplicate]

Question

Possible Duplicate:
Why copy constructor is not called in this case?

Consider the sample program below:

#include <iostream>

using namespace std;

class sample
{
    private:
        int x;

    public:
        sample(int a=0) : x(a)
        {
            cout << "default ctor invoked\n";
        }

        sample(const sample& obj)
        {
            cout << "copy ctor invoked\n";
        }

};

int main()
{
    sample s2 = sample(20); //Line1
    sample s3 = 20; //Line2

    return 0;
}

In Line1, first the constructor of sample class is invoked explicitly with the argument 20. Then i expected the copy constructor to be invoked to initialize s2.

In Line2, first the constructor of sample class is invoked implicitly first with the argument 20. Here also i expected the copy constructor to be invoked to initialize s2.

In both cases, the copy constructor is not invoked? Why is this happening? I believe, there is something wrong with my understanding of the invocation of copy constructor. Could someone correct me where i am going wrong?

Answer 1

This is expected. It's called copy elision.

Your expectation is correct, but they made an exception in C++ (for performance) which allows the compiler to treat your expression as direct initialization of one instance while bypassing the copy constructor.