c++ - Determining the Parameter Types of an Undefined Function -

i've learned cannot:

1. take address of undefined function
2. take address of templatized function type fail compile for

but i've learned can call decltype return type of said function

so undefined function:

int foo(char, short); 

i'd know if there's way can match parameter types types in tuple. meta programming question. i'm shooting decltypeargs in example:

enable_if_t<is_same_v<tuple<char, short>, decltypeargs<foo>>, int> bar; 

can me understand how decltypeargs crafted?

for non-overloaded functions, pointers functions, , pointers member functions, doing decltype(function) gives type of function in unevaluated context, , type contains arguments.

so the argument types tuple, need lot of specializations:

// primary function objects template <class t> struct function_args : function_args<decltype(&t::operator()> { };  // normal function template <class r, class... args> struct function_args<r(args...)> {     using type = std::tuple<args...>; };  // pointer non-cv-qualified, non-ref-qualified, non-variadic member function template <class r, class c, class... args> struct function_args<r (c::*)(args...)> : function_args<r(args...)> { };  // + few dozen more in c++14 // + few dozen more on top of noexcept being part of type system in c++17 

with that:

template <class t> using decltypeargs = typename function_args<t>::type; 

this requires write decltypeargs<decltype(foo)>.

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with c++17, have template <auto>, above can be:

template <auto f> using decltypeargs = typename function_args<decltype(f)>::type; 

and you'd decltypeargs<foo> syntax.