c++ - 为什么 set/get_default_resource 使用指针而不是引用？

Question

当使用 C++17 多态分配器和 set/get_default_resource 方法时(参见 https://en.cppreference.com/w/cpp/memory/set_default_resource)，所有资源都通过指针传递/返回，而使用 nullptr 几乎没有意义，而且感觉它可以大部分时间作为引用(如果需要，然后存储为 ref_wrapper)。

这有什么逻辑/风格上的原因吗？

Answer 1

正如您链接的页面所引用的那样，set_default_resource() 确实接受一个空指针并且在这种情况下具有特定的行为:

If r is not null, sets the default memory resource pointer to r; otherwise, sets the default memory resource pointer to std::pmr::new_delete_resource().

初始论文，N3525 , 也引用了理由:

We have found it is convenient to use nullptr as a surrogate for the “default-default” handler in various interfaces. The use here simply provides consistency and makes it easy to reset the default resource to its initial state.

和后续修订，N3916 , 状态:

Note that the memory-resource library is designed so that the ShoppingList constructor accepts a pointer to a memory_resource rather than a reference to a memory_resource. It was noted that one common practice is to use references rather than pointers in situations where a null pointer is out of contract. However, there is a more compelling practice of avoiding constructors that take objects by reference and store their addresses. We also want to avoid passing non-const references, as that, too, is usually considered bad practice (except in overloaded operators).