Variadic template to define a typedef (using C++11)

Question

I have just defined 4 different typedefs with minimal differences and I'm wondering if there's way to use templates to do this more efficiently.

My typedef is of the form: typedef Type1 (*pf)(Type2, Type3, ...)

How do I template this typedef?

Only Type1 is required.

I manually write:

typedef int (*pf)(int)
typedef bool (*pf)()
typedef char (*pf)(bool, int)

I'm looking for something like:

template <Type T1,Type...Rest>
typedef T1 (*pf)(Type...Rest)

Is that correct?

Can you show the 4 typedefs? That would make it easier to answer your question IMO... — Daniel Jour, May 18 '17 at 21:35
@BoundaryImposition why? That doesn't use variadic templates. — Bob, May 18 '17 at 21:48
@Adrian: Hence the "almost". But it does show you the right tool and how to use it. Going from "templates" to "variadic templates" is a step unrelated to type aliases. You should learn to break down your problems into smaller chunks! — Lightness Races in Orbit, May 18 '17 at 22:09

SergeyA · Accepted Answer · 2017-05-18T21:47:30.150

5

Yes, sure, two lines (could be single line depending on your code style):

template<class T, class... X>
using fptr_t = T (*)(X...);

This employs a technique which is called alias template: http://en.cppreference.com/w/cpp/language/type_alias

Alias template is akin to class template in a sense that it doesn't define new type (like type alias does), instead, it defines a template for defining new types. When used with different types, it gives you type definition based on this template. This is C++11 feature.

edited May 18 '17 at 21:47

answered May 18 '17 at 21:43

SergeyA

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There you go. Again, this answer could do with the basic explanation that `using` is the new hotness for type aliases, and supports templates... everything falls into place after that. – Lightness Races in Orbit May 18 '17 at 21:44
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By the way, that's two lines ;) – Lightness Races in Orbit May 18 '17 at 21:44
@BoundaryImposition, give me 1 min – SergeyA May 18 '17 at 21:45
So obvious, and yet so far from my mind. I don't know what I was thinking. +1. This is the cleanest way. – user2296177 May 18 '17 at 21:47
@user2296177: The irony is that you used this exact solution within the implementation of your own solution! Well, very nearly, anyway. – Lightness Races in Orbit May 18 '17 at 22:10
@BoundaryImposition so my solution wouldn't work? `template typedef T1 (*pf)(Type...Rest)` – Bob May 19 '17 at 01:57
@Adrian `typedef` aliases cannot be templates. – user2296177 May 19 '17 at 02:23
@BoundaryImposition Indeed, my brain was in automatic mode from having recently dealt with templates that actually required inner types. – user2296177 May 19 '17 at 02:25

score 2 · Answer 2 · answered May 18 '17 at 22:06

You can create an easy-to-read function pointer typedef by deferring to a template class specialised on the function signature:

#include <iostream>


namespace detail {
    // define the template concept
    template<class Sig>
    struct function_ptr;

    // specialise on pointer to function of a given signature    
    template<class Ret, class...Args>
    struct function_ptr<Ret (Args...)>
    {
        using type = Ret (*)(Args...);
    };
}

// defer to template specialisation    
template<class Sig>
using function_ptr = typename detail::function_ptr<Sig>::type;

int test1(int) { return 0; }
bool test2() { return false; }
char test3(bool, int) { return 'a'; }

int main()
{

    using pfi = function_ptr <int (int)>;
    using pfv = function_ptr <bool ()>;
    using pfbi = function_ptr <char (bool, int)>;

    auto pt1 = pfi(test1);
    auto pt2 = pfv(test2);
    auto pt3 = pfbi(test3);

    std::cout << pt1(100) << std::endl;
    std::cout << pt2() << std::endl;
    std::cout << pt3(true, 100) << std::endl;
}

I don't like the reuse of `function_ptr`, namespace notwithstanding. Isn't it standard-led convention to call it something like `function_ptr_t` here? — Lightness Races in Orbit, May 18 '17 at 22:11
@BoundaryImposition oops, you're right. It's a poor choice of name. — Richard Hodges, May 18 '17 at 22:19

Variadic template to define a typedef (using C++11)

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