主页 / coding / 问题 / 77324309
Accepted
canellas
Asked: 2023-10-19 21:39:50 +0800 CST

如何在 C++ 20 中定义使用“概念”的“概念”？

  • 772

我正在学习conceptsC++ 20，并提出需要concept使用先前定义的concept.

因此，在下面的示例中，我预计f<pub_b>()会生成编译器错误，因为events_publishedinpub_b不满足publisher概念，因为ev_2不满足event概念。

看来在概念event中使用概念publisher并没有什么效果。

g++ version报告

g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

我究竟做错了什么？

#include <concepts>
#include <tuple>

template <typename t>
concept event = requires {
  std::default_initializable<t>;
  std::copy_constructible<t>;
  std::move_constructible<t>;
};

struct ev_1 {
  ev_1() = default;
  ev_1(const ev_1 &) = default;
  ev_1(ev_1 &&) = default;
};

struct ev_2 {
  ev_2() = delete;
  ev_2(const ev_2 &) = default;
  ev_2(ev_2 &&) = default;
};

template <typename t>
concept publisher = requires {
  typename t::events_published;

  requires[]<std::size_t... t_idx>(std::index_sequence<t_idx...>) {
    return ((event<typename std::tuple_element_t<
                 t_idx, typename t::events_published>>)&&...);
  }
  (std::make_index_sequence<std::tuple_size_v<typename t::events_published>>());
};

struct pub_a {
  using events_published = std::tuple<ev_1>;
};

struct pub_b {
  using events_published = std::tuple<ev_2>;
};

template <publisher t_publisher> void f() {}

int main() {
  f<pub_a>();

  f<pub_b>();

  return 0;
}
c++

2 个回答

  1. 无需requires为您的概念使用 - 子句，因为它通常用于检查表达式的有效性。只是简单地

    #include <concepts>

template <typename t>
concept event = std::default_initializable<t> &&
                std::copy_constructible<t>;

    正如copy_constructible已经保证的那样move_constructible。如果您还需要可复制分配的类型，生活会更容易

    template <typename t>
concept event = std::semiregular<t>;
    • 6
  2. Best Answer

    需要event concept更多requires：

    template <typename t>
concept event = requires {
    requires std::default_initializable<t>;
    requires std::copy_constructible<t>;
    requires std::move_constructible<t>;
};

    int main() {
    f<pub_a>();

    //f<pub_b>(); // Now gives a compilation error
}
    • 5

相关问题