Returning constructor arguments in function, calls the constructor on return

Question

I ran into some code that surprised me. This compiles in Visual Studio 2012 and runs just fine.

The Description Class

class Description {
public:
    Description(bool valid = false);
    ~Description(void);
};

Main

Description functionThatReturnsADescription()
{
    return 1;
}

int main()
{
    Description theDescription = functionThatReturnsADescription();
}

My confusion is with the functionThatReturnsADescription() method; how is it legal to return any object other than Description? If I set it to return 1, it calls the Description constructor with valid = true. If I set it to return false or return 0, it calls the Description constructor with valid = false.

Does this mean a C++ compiler always calls the functions' return type constructor with the arguments of the return statement?? How did I not know that before?! I would have swore this wasn't legal C++.

Answer 1

how is it legal to return any object other than Description?

You can return any type that's implicitly convertible to Description. In this case, any numeric type can be converted to bool via standard conversions, then to Description via the non-explicit constructor.

You could prevent this conversion by declaring the constructor explicit, if you like. It's a good idea to make that a habit, to avoid unexpected conversions like this one.

Does this mean a C++ compiler always calls the functions' return type constructor with the arguments of the return statement??

If the types don't match, and there is a suitable non-explicit constructor to use for the conversion, yes.

How did I not know that before?!

C++ is a complicated language. After fifteen years, I still keep finding things I didn't know.

Answer 2

1 gets implicitly converted to true, so the constructor accepting a bool is chosen.

To prevent this implicit conversion, you can mark the constructor as explicit, in which case only

return Description(1);

would work.

Answer 3

The return value 1 gets implicitly converted to boolean true which is then passed as an argument to the constructor. A Description object gets created implicitly using the constructor Description(bool valid = false);.

It is to be noted that two implicit conversions are happening here: 1 → true → Description. You can prevent the second conversion by making the constructor explicit. Change the declaration of the constructor to

explicit Description(bool valid = false);

However, the first conversion would still happen i.e. one can still do return Description(1);.

Refer: What does the explicit keyword in C++ mean?