template of callable (callback) as class template parameter - c++

Question

I would like to achieve something like this:
template<class IT>
size_t foo(IT begin,IT end) {return end-begin;}
template<template (class) class FOO>
class BAR
{
public:
any_container<any_type> container;
size_t call_foo
{
FOO<any_container<any_type>::iterator>(container.begin(), container.end());
}
};
Moreover, I want to be able to pass function, lambda or function object as FOO.
Probably std::function should be used here but it is not possible to declare std::function<size_t(T,T)> with arbitrary type T.
And I definitely don't want to specify type of inner container or its iterator on template BAR argument list.
Is there any way to solve this in elegant way?

Answer 1

Based on your comment I think you are looking for something like this:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <utility>
#include <vector>
struct Sender {
template<typename Iter>
std::size_t operator()(Iter begin, Iter end) {
for(Iter cur = begin; cur != end; ++cur) {
// your low-level send stuff
}
return (end - begin);
}
};
struct Receiver {
template<typename Iter>
std::size_t operator()(Iter begin, Iter end) {
for(Iter cur = begin; cur != end; ++cur) {
// your low-level receive stuff
}
return (end - begin);
}
};
template<typename Fn>
struct Bar
{
protected:
Fn _fn;
public:
Bar(Fn fn) : _fn(std::move(fn)) { }
template<typename Container>
std::size_t call_fn(Container & container)
{
return _fn(std::begin(container), std::end(container));
}
};
template<typename Fn>
auto create_bar(Fn && fn) -> Bar<typename std::remove_reference<Fn>::type> {
return { std::forward<Fn>(fn) };
}
Usage is simple:
template<typename Iter>
std::size_t my_foo(Iter begin, Iter end) {
return (end - begin);
}
int main(int argc, char ** argv) {
typedef typename std::vector<int>::iterator iter;
// functors
auto functor = create_bar(Sender());
// lambdas
auto lambda = create_bar([](iter begin, iter end) -> std::size_t { return (end - begin); });
// plain old functions
auto function = create_bar(&my_foo<iter>);
std::vector<int> tmp = { 0, 1, 2, 5};
std::cout << "Functor: " << functor.call_fn(tmp) << std::endl;
std::cout << "Lambda: " << lambda.call_fn(tmp) << std::endl;
std::cout << "Function: " << function.call_fn(tmp) << std::endl;
return 0;
}
Live example here