c++constructorexplicitexplicit-constructor

What is the purpose of the explicit keyword on a multi-argument constructor?

I recently came across some weird looking class that had three constructors:

class Class
{
    public:
        explicit Class(int );

        Class(AnotherClass );

        explicit Class(YetAnotherClass, AnotherClass );

    // ...
}

This doesn't really make sense to me - I thought the explicit keyword is to protect compiler chosen construction from a foreign type.

Is this allowed? If it it, what does it mean?

Solution

  • In C++11 multi-parameter constructors can be implicitly converted to with brace initialization.

    However, before C++11 explicit only applied to single-argument constructors. For multiple-argument constructors, it was ignored and had no effect.