Template function binding variadic number of tuple's elements to another function as arguments

Question

Is there any way in c++11/14 to write variadic template function like this:

template <typename ReturnType, typename Args...>
std::function<ReturnType()> bindArgumentsFromTuple
                                  (std::function<ReturnType(Args... args)> f,
                                  const std::tuple<Args...>& t)

that binds elements of tuple t to function f as arguments (such that the number of elements and its types are identical in function arguments and tuple)?

Example of usage:

void dummy_print(int i, double d)
{
    std::cout << i << "," << d << '\n';
}
void dummy_print2(int i, double d, std::string& s)
{
    std::cout << i << "," << d <<  "," << s << '\n';
}

int main() {
    std::tuple<int,double> t(77,1.1);
    std::tuple<int,double,std::string> t2(1,2.0,"aaa");
    auto f1 = bindArgumentsFromTuple(dummy_print, t);
    f1();
    auto f2 = bindArgumentsFromTuple(dummy_print2, t2);
    f2();

    return 0;
}

Columbo · Accepted Answer · 2016-01-12 13:37:35Z

3

template <typename F, typename T, std::size_t... indices>
auto bindTuple(F&& f, T&& t, std::index_sequence<indices...>) {
    return std::bind(std::forward<F>(f), std::get<indices>(std::forward<T>(t))...);
}

template <typename F, typename T>
auto bindTuple(F&& f, T&& t) {
    return bindTuple(std::forward<F>(f), std::forward<T>(t),
                   std::make_index_sequence<std::tuple_size<std::decay_t<T>>{}>{});
}

The above will support anything tuple-esque, i.e. std::array, std::tuple, etc., while regarding placeholder arguments. Demo.

answered Jan 12, 2016 at 13:37

Columbo

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6 Comments

attuda Over a year ago

Thank you! It is my one more knowledge gap in c++: std::index_sequence :)

David G Over a year ago

In the first overload wouldn't you be forwarding t (potentially moving it) multiple times?

Columbo Over a year ago

@0x499602D2 I'm forwarding it so std::get can forward the tuple's elements accordingly, avoiding copies whenever possible

T.C. Over a year ago

With STL's usual caveat about bind doing extra magic that may or may not be desirable.

Columbo Over a year ago

@T.C. You mean INVOKE?

|

Jarod42 · Accepted Answer · 2016-01-12 13:37:24Z

1

With std::index_sequence, you may do something like:

template <typename ReturnType, typename ...Args, std::size_t ... Is>
std::function<ReturnType()>
bindArgumentsFromTuple_impl(std::function<ReturnType(Args...)> f,
                       const std::tuple<Args...>& t, std::index_sequence<Is...>)
{
    return [=]() { return f(std::get<Is>(t)...);};
}

template <typename ReturnType, typename ...Args>
std::function<ReturnType()>
bindArgumentsFromTuple(std::function<ReturnType(Args...)> f,
                       const std::tuple<Args...>& t)
{
    return bindArgumentsFromTuple_impl(f, t, std::index_sequence_for<Args...>{});
}

Live Demo

answered Jan 12, 2016 at 13:37

Jarod42

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5 Comments

Columbo Over a year ago

std::function? Seriously?

Jarod42 Over a year ago

I kept OP's signature, but your are right, your way is better.

Columbo Over a year ago

The OP accepts strings with non-const references; I doubt he knows what's best for him :-)

attuda Over a year ago

Thank you! As I wrote to @Colombo, it is my another knowledge gap in c++: std::index_sequence :)

Jarod42 Over a year ago

@attuda: You can't accept several answers, but you can up vote several answers.

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