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(); 

Answer 2

An answer to my question was proposed in this paper for the next upcoming C++ standard:

• https://isocpp.org/files/papers/n3601.html

This syntax was proposed:

template<using typename T, T t> struct some_struct { /* ... */ };  some_struct<&A::f> x; 

The need for a new syntactical construct indicates that you cannot do that by now.

I hope n3601 will be accepted. :-)