Initialize class containing a std::function with a lambda

Question

I created a template class containing a std::function as a member the following way:

template<typename Ret, typename... Args>
class Foo
{
private:
    std::function<Ret(Args...)> _func;

public:
    Foo(const std::function<Ret(Args...)>& func):
        _func(func)
    {}
};

In order not to have to specify the arguments and return type of the passed function, I created some make_foo overloads:

template<typename Ret, typename... Args>
auto make_foo(Ret (&func)(Args...))
    -> Foo<Ret, Args...>
{
    return { std::function<Ret(Args...)>(func) };
}

template<typename Ret, typename... Args>
auto make_foo(const std::function<Ret(Args...)>& func)
    -> Foo<Ret, Args...>
{
    return { func };
}

However, I was unable to create a make_foo overload that takes a lambda as parameter:

template<typename Ret, typename... Args>
auto make_foo(??? func)
    -> Foo<Ret, Args...>
{
    return { std::function<Ret(Args...)>(func) };
}

I just can't find a way to have the return type and argument types automatically deduced from the lambda. Is there an idiomatic way to solve such a problem?

Answer 1

Ok, so I thought I would die, but I finally managed to do it ç_ç

First, I used the usual indices. Since I do not have the official ones, I used old indices I wrote some months ago:

template<std::size_t...>
struct indices {};

template<std::size_t N, std::size_t... Ind>
struct make_indices:
    make_indices<N-1, N-1, Ind...>
{};

template<std::size_t... Ind>
struct make_indices<0, Ind...>:
    indices<Ind...>
{};

Then, I used some function traits found somewhere on StackOverflow. They are nice, and I think that they are equivalent to the Boost library linked in the comments:

template<typename T>
struct function_traits:
    function_traits<decltype(&T::operator())>
{};

template<typename C, typename Ret, typename... Args>
struct function_traits<Ret(C::*)(Args...) const>
{
    enum { arity = sizeof...(Args) };

    using result_type = Ret;

    template<std::size_t N>
    using arg = typename std::tuple_element<N, std::tuple<Args...>>::type;
};

Then, I was able to write a proper make_foo function and it implementation function, since both are required to use indices. Be careful, it's plain ugly:

template<typename Function, std::size_t... Ind>
auto make_foo_(Function&& func, indices<Ind...>)
    -> Foo<
        typename function_traits<typename std::remove_reference<Function>::type>::result_type,
        typename function_traits<typename std::remove_reference<Function>::type>::template arg<Ind>...>
{
    using Ret = typename function_traits<typename std::remove_reference<Function>::type>::result_type;
    return { std::function<Ret(typename function_traits<typename std::remove_reference<Function>::type>::template arg<Ind>...)>(func) };
}

template<typename Function, typename Indices=make_indices<function_traits<typename std::remove_reference<Function>::type>::arity>>
auto make_foo(Function&& func)
    -> decltype(make_foo_(std::forward<Function>(func), Indices()))
{
    return make_foo_(std::forward<Function>(func), Indices());
}

The code is somehow ugly and unreadable, but it definitely works. Hope it does not rely on some implementation-defined behaviour now. Also, thanks all for your advice, it helped! :)

int main()
{
    auto lambda = [](int i, float b, long c)
    {
        return long(i*10+b+c);
    };

    auto foo = make_foo(lambda);
    std::cout << foo(5, 5.0, 2) << std::endl; // 57, it works!
}

---

And here is the live example :)

Answer 2

I have an example that works with mutable lambdas. I can't quite figure out how to get the CV member qualification right.

First, here's the function template we're after:

#include <functional>

template <typename R, typename ...Args>
void foo(std::function<R(Args...)> f)
{ }

Now we'll let a function template bar take an arbitrary lambda and call the right version of foo, by inspecting the type of the lambda's operator():

#include <type_traits>

template <typename> struct remove_member;

template <typename C, typename T>
struct remove_member<T C::*>
{ using type = T; };

template <typename F>
void bar(F f)
{
    using ft = decltype(&F::operator());
    foo(std::function<typename remove_member<ft>::type>(f));
}

Example:

int q;
bar([&](int a, int b) mutable -> int { q = a + b; return q / b; });

---

You can use normal, const lambdas with this modified trait, though I don't like having to spell the function type out:

template <typename C, typename R, typename ...Args>
struct remove_member<R (C::*)(Args...) const>
{ using type = R(Args...); };

I thought it might work with the original code if I use typename std::remove_cv<T>::type, but at least on GCC this doesn't work because of some strange __attribute__((const)) that's set on the lambda's operator type which seems to interfere with the template specialization.