c++ - Pointer to class member as a template parameter

Question

I want to use a pointer to a class member as a template parameter as in:

template <class Class, class Result, Result Class::*Member>
struct MyStruct {
    // ...
};

Using this struct like MyStruct<SomeClass, SomeResult, &SomeClass::value> variable works just fine, but I don't like that I have to specify SomeClass and SomeResult.

I would like to use MyStruct<&SomeClass::value> variable if that is possible, but without losing the ability to pass any class and have any result type.

I tried the following, but the syntax is illegal:

template <class Class, class Result>
template <Result Class::*Member>
struct MyStruct {
    // ...
};

error: too many template-parameter-lists

I tried using a helper function (that does actually work in Clang but is refused by GCC):

template <class Class, class Result>
static constexpr auto makeMyStruct(Result Class::*member) ->
MyStruct<Class, Result, member> {
    // ...
}

error: use of parameter `member' outside function body
error: template argument 3 is invalid

Is it possible to have a simple MyStruct<&SomeClass::value>, and if so, how?

Related question that did not solve my question:

• Pointer to class member as template parameter
• C++0x error with constexpr and returning template function

Answer 1

In c++17, with the addition of auto in template arguments (P0127), I think you can now do:

template<auto value>
struct MyStruct {};

template<typename Class, typename Result, Result Class::* value>
struct MyStruct<value> {
    // add members using Class, Result, and value here
    using containing_type = Class;
};

typename MyStruct<&Something::theotherthing>::containing_type x = Something();