Quesion on explicit copy constructors.

[jimmyi]

i wonder why the following code can not be compiled.

// compiler: gcc 3.3.1
#include <iostream>
using namespace std;

class a {
public:
a(){}
explicit a(const a& _a) {}
};

a fun()
{
return a(); // what's happened here?
}

int main(){}


Anyboby can help to rescue? thanks!!!

[jsmith]

Never, ever, make a copy constructor explicit, unless you want to jump through hoops to fix the compile errors the above code gives you.

In order to return an "a" from a function, the copy constructor has to be called .... implicitly. There is no
way to return an "a" by explicitly calling the copy constructor. You would have to return either a reference
to a static instance of "a" or a pointer to a dynamically allocated instance of "a".

By declaring the copy constructor explicit, you've told the compiler that it cannot call your copy constructor
implicitly ... ever.