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.

Answer 2

in first line it does not invoke copy constructor because you do not copy an object. You are assigning one object to other. C++ provide default = operator which perform shallow copy. And this is invoked implicitly. The constructor is called for right hand object and default constructor is invoked for the left hand object. After that default = operator is invoked.

For line 2, it use constructor that takes int parameters that you define. It is actually converter constructor because it takes an integer and creates object of your class. Thats why c++ use this as a converter constructor and when you try to assign an integer to your object, c++ invokes inplicitly this converter constructor.

I hope this helps you to understand.