Question

I’m running into an issue with a variadic function template. I need to examine each element of a parameter pack, package the element, then stuff all the packaged elements into a tuple and return that. Here’s the general idea of what I’d like to do (return types are just placeholders, not sure what they’d be):

template<typename A>
sometype func_helper(A a) {
    //examine a, depending on type, do different stuff with it.
    return modified_a;
}

template<typename... Args>
tuple<sometypes...> func(Args... args) {
    return make_tuple(func_helper...(args));
}

Any ideas?

Answer 1

You can use deduced return type. Sadly it has code reduplication:

#include <iostream>
#include <tuple>

template<typename A>
int func_helper(A ) {
    //examine a, depending on type, do different stuff with it.
    return 1;
}

char func_helper(double) {
    return 'A';
}

template<typename ...Args>
auto func(Args... args) -> decltype(std::make_tuple(func_helper(args)...)) {
    return std::make_tuple(func_helper(args)...);
}

int main()
{
    auto a = func(1, 3.4);
    std::cout << std::get<0>(a) << ' ' << std::get<1>(a) << '\n';
}