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Apply tuple to a function

12345678910111213141516171819202122232425262728293031#include <cstddef> #include <tuple> #include <type_traits> #include <utility> template<typename F, typename Tuple, size_t ...S > decltype(auto) apply_tuple_impl(F&& fn, Tuple&& t, std::index_sequence<S...>) { return std::forward<F>(fn)(std::get<S>(std::forward<Tuple>(t))...); } template<typename F, typename Tuple> decltype(auto) apply_from_tuple(F&& fn, Tuple&& t) { std::size_t constexpr tSize = std::tuple_size<typename std::remove_reference<Tuple>::type>::value; return apply_tuple_impl(std::forward<F>(fn), std::forward<Tuple>(t), std::make_index_sequence<tSize>()); } int do_sum(int a, int b) { return a + b; } int main() { int sum = apply_from_tuple(do_sum, std::make_tuple(10, 20)); }

This sample is licensed under the CC0 Public Domain Dedication.

Intent

Unpack a tuple as the arguments of a function.

Description

The apply_from_tuple function template on lines 12–21 returns the result of applying the function fn to the values stored in the std::tuple t. On lines 15–16, we store the size of t in tSize, which is declared constexpr so that it can be evaluated at compile-time. On lines 18–20, we call apply_tuple_impl passing t, fn and an std::index_sequence which carries a parameter pack containing a sequence of integers from 0 to tSize - 1.

The apply_tuple_impl function template on lines 6–10 returns the result of applying the function fn using event element of the tuple t as arguments (on line 9). To do this, we expand the parameter pack carried by the std::index_sequence and apply std::get to the tuple for each integer in the sequence. This way, all the elements of t are expanded and passed to the function.

Note: a std::apply function has been proposed as part of the Library Fundamentals TS for future standardization.

Contributors

  • Joseph Mansfield
  • Marco Arena

Last Updated

11 February 2016

Source

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