use the signature of a generic function dependent on a non-type template argument as template template argument

问题

The small program below, which compiles and runs, allows to bridge a runtime variable index of type unsigned with a set of template functions having one template argument J of type unsigned.

In case further clarifications are needed, this is better explained in this question.

The auxiliary functions I wrote use a template template argument, to infer as much info as possible from the original function. The problem is that I could not find a better way to define the template template argument FunWrap other than creating the 2 wrappers wrap_foo and wrap_zoo, whereas I would have liked to use as template template argument directly foo and zoo. Is there a way to do that?

#include <iostream>
#include <utility>
#include <array>
using namespace std;

// boiler plate stuff

template <template <unsigned J> typename FunWrap, unsigned... Is>
decltype(auto) get_fun_ptr_aux(std::integer_sequence<unsigned, Is...>, unsigned i)
{
    typedef decltype(&FunWrap<1>::run) FunPtr;
    constexpr static std::array<FunPtr, sizeof...(Is)> fun_ptrs = { &FunWrap<Is>::run... };
    return fun_ptrs[i];
}

template <template <unsigned J> typename FunWrap, unsigned N>
decltype(auto) get_fun_ptr(unsigned i)
{
    return get_fun_ptr_aux<FunWrap>(std::make_integer_sequence<unsigned, N>{}, i);
}

// template functions to be bridged with runtime arguments
// two functions with same template arguments but different signature

template <unsigned J>
void foo() {  cout << J << "\n"; }

template <unsigned J>
double zoo(double x) { return x + J; }

// 1 wrapper per each function

template <unsigned J>
struct wrap_foo {
    static void *run() { foo<J>(); }  // same signature as foo
};

template <unsigned J>
struct wrap_zoo {
    static double run(double x) { return zoo<J>(x); } // same signature as zoo
};

int main()
{
    unsigned index = 5;
    (*get_fun_ptr<wrap_foo,10>(index))();
    cout << (*get_fun_ptr<wrap_zoo,10>(index))(3.5) << "\n";
    return 0;
}

回答1:

I would have liked to use as template template argument directly foo and zoo. Is there a way to do that?

As foo and zoo are template functions, I'm afraid the answer is no. C++ is notoriously bad at dealing with overload sets/templates. Example: Disambiguate overloaded member function pointer being passed as template parameter

来源：https://stackoverflow.com/questions/47835596/use-the-signature-of-a-generic-function-dependent-on-a-non-type-template-argumen