Question

say we have a struct template taking a class , and a pointer on a member on this class:

struct A<A,&A::a>

I can’t declare template like that

template<class T,class U>
struct{};

I have to write

template<class T,typename T::type var>
struct{};

why &A::a can’t be bind into a simple typename T syntax ? Before to be a pointer on a member, &A::a is a type so we could expect that a simple typename T works but that is not the case

Answer 1

&A::a is a value, not a type. So the template declaration makes no sense.

Here’s how it might work:

template <typename A, int A::* Ptr> struct Foo { };

struct Bar { int n; };

int main()
{
    Foo<Bar, &Bar::n> f;
}

Or, more generally,

template <typename T, typename U, U T::* Ptr> struct Foo { };

Foo<Bar, int, &Bar::n> f;