c++ - 为什么 `polymorphic_allocator` 采用 `memory_resource` 指针而不是引用？

Question

C++17 标准说:

[mem.poly.allocator.ctor]

polymorphic_allocator(memory_resource* r);

• Requires: r is non-null.

• Effects: Sets memory_­rsrc to r.

• Throws: Nothing.

• [ Note: This constructor provides an implicit conversion from memory_­resource*. — end note ]

接受 memory_resource* 有什么意义？而不是 memory_resource&如果 “需要” 子句提到 r必须是非空的？

Bloomberg¹ 风格指南鼓励接受将被指针而不是引用改变的参数，以便调用方的 & 符号成为改变的视觉标记。但是，标准中没有这样的先例。

是什么原因r被当作​​指针而不是引用？

---

¹ pmr由于该公司使用多态分配器模型，因此在彭博社的大力参与下实现了标准化。

Answer 1

N3916 :

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).