Originally, I had a C++ class A with a templated class member function member(). Because member() is a performance hotspot, it has multiple internal implementations. Depending on some runtime conditions, the public function member() selects one of them by instantiating the templated member function memberImpl().

class A
{
    template <typename T>
    void memberImpl()
    {
    }

    void member()
    {
        // if (a) {
            memberImpl<float>();
        //}
        // else {
            memberImpl<int>();
        // }
    }
};

It worked, but I noticed that the same check is necessary for multiple member functions, thus, instead of repeating the same check in each member function, I thought it would be a good idea to create a new second-order template dispatch that accepts the original first-order template as a template template for doing the check and instantiating the underlying templates.

class A
{
    template <typename T>
    void memberImpl()
    {
    }

    template <template <typename> typename Impl>
    void dispatch()
    {
        // if (a) {
            Impl<float>();
        //}
        // else {
            Impl<int>();
        // }
    }

    void member()
    {
        dispatch<memberImpl>(); 
    }
};

I believe this substitution is possible during compile-time, and it’s within the capability of C++ templates. Unfortunately it’s an invalid C++ program according to GCC and clang:

demo.cpp: In member function ‘void A::member()’:
demo.cpp:23:37: error: no matching function for call to ‘A::dispatch<((A*)this)->A::memberImpl>()’
   23 |                 dispatch<memberImpl>();
      |                 ~~~~~~~~~~~~~~~~~~~~^~
demo.cpp:11:14: note: candidate: ‘template<template<class> class Impl> void A::dispatch()’
   11 |         void dispatch()
      |              ^~~~~~~~
demo.cpp:11:14: note:   template argument deduction/substitution failed:
demo.cpp:23:37: error: type/value mismatch at argument 1 in template parameter list for ‘template<template<class> class Impl> void A::dispatch()’
   23 |                 dispatch<memberImpl>();
      |                 ~~~~~~~~~~~~~~~~~~~~^~
demo.cpp:23:37: note:   expected a class template, got ‘((A*)this)->A::memberImpl’

It seems that I believe memberImpl is a template, but the compiler says it’s actually a function. I tried to add the keyword template to different places but was unsuccessful.

What is the correct C++ syntax to achieve my goal?