Optimal way to access std::tuple element in runtime by index

Question

I have function at designed to access std::tuple element by index specified in runtime

template

        

Answer 1

Assuming you pass something similar to a generic lambda, i.e. a function object with an overloaded function call operator:

#include 

struct Func
{
    template
    void operator()(T p)
    {
        std::cout << __PRETTY_FUNCTION__ << " : " << p << "\n";
    }
};

The you can build an array of function pointers:

#include 

template struct seq {};
template struct gen_seq : gen_seq {};
template struct gen_seq<0, Is...> : seq {};

template
void apply_one(T& p, F func)
{
    func( std::get(p) );
}

template
void apply(T& p, int index, F func, seq)
{
    using FT = void(T&, F);
    static constexpr FT* arr[] = { &apply_one... };
    arr[index](p, func);
}

template
void apply(T& p, int index, F func)
{
    apply(p, index, func, gen_seq::value>{});
}

Usage example:

int main()
{
    std::tuple t{1, 2.3, 4, 5.6};
    for(int i = 0; i < 4; ++i) apply(t, i, Func{});
}

---

clang++ also accepts an expansion applied to a pattern that contains a lambda expression:

static FT* arr[] = { [](T& p, F func){ func(std::get(p)); }... };

(although I've to admit that looks really weird)

g++4.8.1 rejects this.