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Accepted
Juergen
Asked: 2024-09-19 19:37:10 +0800 CST

未定义模板 std::tuple_element 的隐式实例化

  • 772

片段，同样在godbolt上

#include <tuple>
#include <utility>

template <typename Tuple, std::size_t... Ints>
auto select_tuple(Tuple&& tuple, std::index_sequence<Ints...>) {
    return std::tuple<std::tuple_element_t<Ints, Tuple>...>(
        std::get<Ints>(std::forward<Tuple>(tuple))...);
}

int main() {
    std::tuple<int, char, float> t{1, 'x', 2.0};
    auto t2 = select_tuple(t, std::index_sequence<0, 2>{});
}

正如您在 godbolt 链接中看到的，编译器抛出了一个错误：

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/utility.h:135:5: error: implicit instantiation of undefined template 'std::tuple_element<0, std::tuple<int, char, float> &>'
  135 |     using tuple_element_t = typename tuple_element<__i, _Tp>::type;

编译器认为它不知道std::tuple_element，但tuple已包含。这里出了什么问题？

c++

1 个回答

  1. Best Answer

    您实际上有std::tuple_element_t<0, std::tuple<int, char, float>&>（注意引用），您需要删除特征中的引用（和 cv 限定符）：

    template <typename Tuple, std::size_t... Ints>
auto select_tuple(Tuple&& tuple, std::index_sequence<Ints...>) {
    return std::tuple<std::tuple_element_t<Ints, std::decay_t<Tuple>>...>(
        std::get<Ints>(std::forward<Tuple>(tuple))...);
}

    演示

    • 4

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