为多个元组的每个第 n 个参数调用 lambda？

Question

我正在尝试编写一个代码，使用从一组可变元组中提取的输入参数调用 lambda。但是，我的尝试没有编译：

#include <iostream>
#include <tuple>
#include <utility>
#include <type_traits>

template <typename ...>
struct first_of;

template <typename T, typename ... Args>
struct first_of<T, Args...> {
    using type = std::decay_t<T>;
};

template <typename T>
struct first_of<T> {
    using type = std::decay_t<T>;
};

template <typename ... T>
using first_of_t = typename first_of<T...>::type;

template <typename Fn, typename... Tuples, std::size_t... Idxs>
void run_impl(Fn&& fn, std::index_sequence<Idxs...>, Tuples... t) {
  auto temp = {(fn(std::get<Idxs>(t)...), true)...};
  (void)temp;
}

template <typename Fn, typename... Tuples>
void run(Fn&& fn, Tuples&&... tuples) {
  run_impl(std::forward<Fn>(fn), std::make_index_sequence<std::tuple_size<first_of_t<Tuples...>>::value>{}, std::forward<Tuples>(tuples)...);
}

int main() {
    auto a = std::make_tuple(1, 2.34, "one");
    auto b = std::make_tuple(32, 5.34, "two");

    auto print = [](auto& f, auto& g) { std::cout << f << ", " << g << std::endl; };
    run(print, a, b);
}

我期待以下输出：

1, 32
2.34, 5.34
one, two

我使用的是 c++14，很遗憾，没有折叠表达式。这是代码的神栓 link：https://godbolt.org/z/G19n5z

Answer 1

最简单的方法是添加另一层间接，让 run_impl 委托给另一个执行实际调用的函数。我冒昧地将您的函数重命名为 call_transposed():

template <std::size_t I, typename Fn, typename... Tuples>
void call_with_nth(Fn&& fn, Tuples&&... t) {
    fn(std::get<I>(std::forward<Tuples>(t))...);
}

template <typename Fn, std::size_t... Idxs, typename... Tuples>
void call_transposed_impl(Fn&& fn, std::index_sequence<Idxs...>, Tuples&&... t) {
  auto temp = {(call_with_nth<Idxs>(fn, std::forward<Tuples>(t)...), true)...};
  (void)temp;
}

template <typename Fn, typename... Tuples>
void call_transposed(Fn&& fn, Tuples&&... tuples) {
  call_transposed_impl(
      std::forward<Fn>(fn),
      std::make_index_sequence<std::tuple_size<first_of_t<Tuples...>>::value>{},
      std::forward<Tuples>(tuples)...);
}

Godbolt link

我不确定为什么你的代码不起作用，但我怀疑 std::get<Idxs>(t)... 试图同时扩展两个包 Idxs 和 t，离开你没有包可以稍后扩展。这段代码一次只处理一个包，从而避免了这个问题。

为多个元组的每个第 n 个参数调用 lambda？

Calling a lambda for each nth argument of multiple tuples?

c++

tuples

template-meta-programming

variadic-templates

c++14