function composition

compose.cpp

#include <utility>

template <typename f1, typename f2>
struct compose_ {
    f1 fun1;
    f2 fun2;

    template <typename... as>
    auto operator()(as&&... args) ->
    decltype(fun1(fun2(std::forward<as>(args)...)))
    {
        return fun1(fun2(std::forward<as>(args)...));
    };
};

template <typename f1, typename f2>
compose_<f1, f2>
compose(f1&& fun1, f2&& fun2)
{
    return { std::forward<f1>(fun1), std::forward<f2>(fun2) };
}

static int
add1(int x)
{
    return x + 1;
}

static int
add2(int x)
{
    return x + 2;
}

int
main()
{
    auto f = compose(add1, add2);
    return f(1);
}