Question

The code below is rejected by VC++ 2012 with “error C2207: ‘A::bar’ : a member of a class template cannot acquire a function type”.

int Hello(int n)
{
    return n;
}

template<class FunctionPtr>
struct A
{
    A(FunctionPtr foo)
        : bar(foo)
    {}

    FunctionPtr bar;
};

int main()
{
    A<decltype(Hello)> a(Hello);

    return 0;
}

Why?

Answer 1

gcc is a bit more friendly regarding this error :

error: field 'A<int(int)>::bar' invalidly declared function type

The simplest solution is to declare bar as a function pointer :

FunctionPtr *bar;

In this case, decltype(Hello) evaluates to int(int) not int(*)(int).