方法作为可变参数模板参数

Question

I'm trying to extend to methods the technique explained here for functions. The problem is the method signature template parameter. For instance, a wrapper for the sin function is created this way:

template<typename Sig, Sig& S> struct wrapper;

template<typename Ret, typename... Args, Ret(&P)(Args...)>
struct wrapper<Ret(Args...), P> {
    // blah
}

and then is instantiated with

wrapper<decltype(sin), sin>

But for a method bool Foo::blah(int) this technique is rejected:

template<class C, typename Sig, Sig& S> struct wrapper;

template<class C, typename Ret, typename... Args, Ret(C::*P)(Args...)>
struct wrapper<Ret(C::)(Args...), P> {
    // blah
}

wrapper<decltype(Foo::blah), &Foo::blah>

So what's the proper syntax?

Answer 1

There is no analogue of free function types for member functions. You need to work with the pointers-to-member-functions directly.

Try something like this:

template <typename C, typename MFP, MFP> struct wrapper;

template <typename C, typename R, typename ...Args, R (C::*MFP)(Args...)>
struct wrapper<C, R (C::*)(Args...), MFP>
{
    // ...
};

Note that this will get a bit verbose if you want to admit all the possible combinations of CV- and rvalue-qualifications.