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July 25, 2023 08:18
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Looping over/dynamic indexing of std::tuple in C++17
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#include <iostream> | |
#include <tuple> | |
#include <type_traits> | |
#include <utility> | |
template <int I> | |
struct constexpr_int{ | |
constexpr operator int() const { return I; } | |
constexpr constexpr_int<I+1> operator++() const { return {}; } | |
}; | |
// f: void(? tuple_element, constexpr_int index) | |
template <class T, class F> | |
void for_each_i(T&& t, F&& f) { | |
auto loop = [&](auto& self, auto i) { | |
f(std::get<i>(t), i); | |
if constexpr (i + 1 < std::tuple_size_v<std::decay_t<T>>) | |
self(self, ++i); | |
}; | |
if constexpr (std::tuple_size_v<std::decay_t<T>> > 0) | |
loop(loop, constexpr_int<0>{}); | |
} | |
// f: void(? tuple_element) | |
template <class T, class F> | |
void for_each(T&& t, F&& f) { | |
for_each_i(std::forward<T>(t), [&](auto&& elem, auto /*index*/){ | |
f(std::forward<decltype(elem)>(elem)); | |
}); | |
} | |
// f: void (? tuple_element) | |
template <class T, class F> | |
void find(T&& tuple, int index, F&& f) { | |
for_each_i(std::forward<T>(tuple), [&](auto&& elem, auto i){ | |
if (i == index) f(std::forward<decltype(elem)>(elem)); | |
}); | |
} | |
int main(int argc, char** argv) { | |
auto t = std::make_tuple("Hello", 3.14f, 42); | |
for_each(t, [](const auto& x){ | |
std::cout << x << ' '; | |
}); | |
std::cout << "\n\n"; | |
for_each_i(t, [](const auto& x, auto i){ | |
std::cout << i << ':' << x << '\n'; | |
}); | |
std::cout << '\n'; | |
find(t, argc, [](const auto& x){ | |
std::cout << x; | |
}); | |
return 0; | |
} | |
// Prints: | |
// Hello 3.14 42 | |
// | |
// 0:Hello | |
// 1:3.14 | |
// 2:42 | |
// | |
// 3.14 | |
// | |
// try it: https://godbolt.org/z/ns6KqKGq3 |
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