Question

consider the following:

class X {
public:
    X(int i) { cout << "X(int i)" << endl; }
    X(const X& x) { cout << "X(const X& x)" << endl; }
};

void main() {
    X x1(1);
    X x2 = X(1);
    X x3 = (X)1;
}

running this code produces this output:

X(int i)
X(int i)
X(int i)

I thought that all of the above three statements are equivalent as the copy CTOR is never called. However, changing X‘s copy CTOR to be private:

class X {
public:
    X(int i) { cout << "X(int i)" << endl; }
private:
    X(const X& x) { cout << "X(const X& x)" << endl; }
};

Will fail to compile (In visual studio 2010) with this error:

cannot access private member declared in class 'X'

So it seems the copy CTOR is involved somehow though I don’t quite understand how.

Thanks

Answer 1

X x1(1);
X x2 = X(1);
X x3 = (X)1;

The reason is that all of these are not exactly equivalent.

First one is direct-initialization, while the second and third is copy-initialization. For copy-initialization, copy-constructor must be public, or else compiler will give error.

Now the question is, if 2nd and 3rd requires the copy-ctor to be public,then why the following output:

X(int i)
X(int i)
X(int i)

This surely says that copy-ctor is never called which is true. Compiler just elided the call to copy-ctor. According to §8.5/14, in such cases, the compiler is permitted to eliminate the need to call copy-constructor. That is why you don’t see copy-ctor being called.

A little inside : in the 2nd and 3rd case, first a temporary is created by calling X(int i), then this temporary was supposed to be passed to the copy-ctor to copy-initialize the object being declared. But the compiler optimizes away this step, eliding the call to copy-ctor.