Question

Say i have the following class :

class A
{
public:
    A() {
    }

    A(int a):_a(a){
    }

    int _a;
};

And the following function :

void someFunc (A a)
{
    cout << a._a;
}

So the following line in the program works fine :

someFunc (5); // Calls A(int a) Constructor.

But the following does not :

someFunc(); //Compile error

One can expect that if it can build A when getting an integer,
why not build one using the default constructor as well, when called with no arguments?

Answer 1

This is because the signature of someFunc() does not match that of void someFunc (A a).

According to C++ standard, Section 13.3.2.3:

• First, to be a viable function, a candidate function shall have enough parameters to agree in number with the arguments in the list.
• A candidate function having more than m parameters is viable only if the (m+1)-st parameter has a default argument

None of this applies in this case, so void someFunc (A a) is not considered viable for the invocation with an empty parameter list.