std::(customization point) 是否调用最合适的重载？

Question

从C++20开始，在[namespace.std]/7中引入了自定义点的概念：

Other than in namespace std or in a namespace within namespace std, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. [ Note: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note ]

注释部分（注意强调的词"qualified"）是否意味着如果std::f是自定义点，std::f将自动为f调用最合适的重载？

一个真实的例子是std::swap, which is a designated customization point. Does this mean since C++20, we can write std::swap(a, b) directly instead of using std::swap; swap(a, b);?

Answer 1

A real example is std::swap, which is a designated customization point. Does this mean since C++20, we can write std::swap(a, b) directly instead of using std::swap; swap(a, b);?

没有。 std::swap本身没有获得任何权力。它仍然只是一个函数模板，所以如果你直接调用它，你就是……直接调用它。没有 ADL 或任何东西。

这里的重点是说如何 自定义点应该被选择。也就是你写：

namespace N { // not std
    void swap(Foo&, Foo&);
}

不是：

namespace std {
    void swap(N::Foo&, N::Foo&);
}

也不是：

namespace std {
    template <>
    void swap(N::Foo&, N::Foo&);
}

---

但是，C++20 确实引入了很多称为 customization point objects which you can use directly do this kind of thing. The CPO for swap is spelled std::ranges::swap 的新东西（同样，所有有用范围的东西都有 CPO...ranges::begin、ranges::end等）。