如何将C ++ 20约束的几种返回类型需求合并为一个返回类型需求？ [英] How can I combine several return type requirements of C++20 constraints into one return type requirement?

问题描述

当前，我已经实现了 Allocator 概念（指提高建议 a>）使用C ++ 20 约束和概念 ：

Currently, I have implemented the Allocator concept (which refers to the Boost proposal) using C++20 constraints and concepts:

#include <concepts>
#include <iterator>

template <class A>
concept allocator =
  std::copy_constructible<A> &&
  std::equality_comparable<A> &&
  requires(A a) {
    { a.allocate(0) } -> std::regular;
    { a.allocate(0) } -> std::constructible_from<std::nullptr_t>;
    { a.allocate(0) } -> std::equality_comparable_with<std::nullptr_t>;
    { a.allocate(0) } -> std::random_access_iterator;
    { *a.allocate(0) } -> std::same_as<typename A::value_type&>;
  };

您可以看到对于同一<$ c，有多个 return-type-requirement $ c>分配函数。有什么方法可以将它们组合成一个单一的 return-type-requirement ，如下所示？

You can see there are several return-type-requirements for the same allocate function. Is there some way to combine them into one single return-type-requirement like the following?

{ a.allocate(0) } -> std::regular &&
                     std::constructible_from<std::nullptr_t> &&
                     std::equality_comparable_with<std::nullptr_t> &&
                     std::random_access_iterator;

推荐答案

对于这种特殊情况，您应该遵循更多概念密切。 allocator_traits< A> :: allocate 的返回类型必须为 allocator_traits< A> :: pointer 。这就是您应该测试的内容。 allocator_traits< A> :: pointer 必须满足连续迭代器和可为空指针的各种约束。

For this particular case, you should follow the concept more closely. The return type of allocator_traits<A>::allocate is required to be allocator_traits<A>::pointer. So that's what you should test. It is allocator_traits<A>::pointer that must satisfy the various constraints of contiguous iterator and nullable pointer.

因此代码应该看起来像这样：

So the code should look like this:

template<typename P>
concept nullable_pointer =
  std::regular<P> &&
  std::convertible_to<std::nullptr_t, P> &&
  std::assignable_from<P&, std::nullptr_t> &&
  std::equality_comparable_with<P, std::nullptr_t>
  

template<typename P>
concept allocator_pointer =
  nullable_pointer<P> &&
  std::contiguous_iterator<P>;

template<typename A>
concept allocator =
  std::copy_constructible<A> &&
  std::equality_comparable<A> &&
  requires(A a)
  {
    { a.allocate(0) } -> allocator_pointer;
  };

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