Passing a variadic function as argument

Question

Consider this working code:

#include <iostream>
#include <utility>
#include <array>

template <typename... Args>
void foo (Args&&... args) {
    const auto v = {args...};
    for (auto x : v) std::cout << x << ' ';     std::cout << '\n';
}

template <typename> struct Foo;

template <std::size_t... Is>
struct Foo<std::index_sequence<Is...>> {
    template <typename Container>
    static void execute (const Container& v) {
        foo(v[Is]...);
    }
};

template <std::size_t N>
void fooArray (const std::array<int, N>& a) {
    Foo<std::make_index_sequence<N>>::execute(a);
}

int main() {
    fooArray<6>({0,1,2,3,4,5});  // 0 1 2 3 4 5
}

I want to now generalize the Foo struct like so:

#include <iostream>
#include <utility>
#include <array>

template <typename... Args>
void foo (Args&&... args) {
    const auto v = {args...};
    for (auto x : v) std::cout << x << ' ';     std::cout << '\n';
}

template <typename> struct Foo;

template <std::size_t... Is>
struct Foo<std::index_sequence<Is...>> {
    template <typename Container, typename F>  // *** Modified
    static void execute (const Container& v, F f) {
        f(v[Is]...);
    }
};

template <std::size_t N>
void fooArray (const std::array<int, N>& a) {
    Foo<std::make_index_sequence<N>>::execute(a, foo);
}

int main() {
    fooArray<6>({0,1,2,3,4,5});
}

But I get a compile error (from GCC 4.9.2) that F cannot be deduced. How do I achieve this?

Answer 1

foo is a family of overloads, and so the foo is ambiguous.
(even foo<int, int> is, as it may have additional type too).

You may force expected type function as follow:

template <std::size_t... Is>
struct Foo<std::index_sequence<Is...>> {
    template <typename Container>
    static void execute (const Container& v, void (*f)(decltype(v[Is])&...)) {
        f(v[Is]...);
    }
};

Live example

An alternative is to wrap function foo into a class:

class FooCaller
{
public:
    template <typename... Args>
    void operator () (Args&&... args) const {
        const auto v = {args...};
        for (auto x : v) std::cout << x << ' ';     std::cout << '\n';
    }

};

and keep your implementation:

Live Demo