Logic of std::is_base_of in C++ 11

Question

While going through some of the C++ concepts I stumbled upon std::is_base_of logic.

Googling around for logic produced the below code, but I am not able to understand it.

Could somebody explain me how it works?

template<typename D, typename B>
class IsDerivedFromHelper
{
    class No { };
    class Yes { No no[3]; };

    static Yes Test( B* );
    static No Test( ... );
public:
    enum { Is = sizeof(Test(static_cast<D*>(0))) == sizeof(Yes) };

};


template <class C, class P> 
bool IsDerivedFrom() {
    return IsDerivedFromHelper<C, P>::Is;
}

Answer 1

When B is a base class of D , the call Test(static_cast<D*>(0)) resolves to Yes Test(B*) . Otherwise, it resolves to No Test(...) .

If B is a base class of D , the value of sizeof(Test(static_cast<D*>(0))) is sizeof(Yes) . Otherwise, it is equal to sizeof(No) .

Yes and No are defined such that sizeof(Yes) will never be equal to sizeof(No) .

If B is a base class of D ,

sizeof(Test(static_cast<D*>(0))) == sizeof(Yes)

evaluates to true . Otherwise it evaluates to false .