是否将shared_ptr的删除器存储在自定义分配器分配的内存中? [英] Is a shared_ptr's deleter stored in memory allocated by the custom allocator?

问题描述

说我有一个shared_ptr，带有一个自定义分配器和一个自定义删除器.

Say I have a shared_ptr with a custom allocator and a custom deleter.

我在标准中找不到任何内容讨论删除器的存储位置:它没有说自定义分配器将用于删除器的内存，也没有说它是不会.

I can't find anything in the standard that talks about where the deleter should be stored: it doesn't say that the custom allocator will be used for the deleter's memory, and it doesn't say that it won't be.

这是未指定的，还是我只是缺少一些东西?

Is this unspecified or am I just missing something?

推荐答案

util.smartptr.shared.const/9在C ++ 11中:

util.smartptr.shared.const/9 in C++ 11:

效果:构造一个拥有对象p和删除器d的shared_ptr对象.第二个和第四个构造函数应使用a的副本为内部使用分配内存.

Effects: Constructs a shared_ptr object that owns the object p and the deleter d. The second and fourth constructors shall use a copy of a to allocate memory for internal use.

第二个和第四个构造函数具有以下原型:

The second and fourth constructors have these prototypes:

template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

在最新草案中，util.smartptr.shared.const/10等效于我们的目的:

In the latest draft, util.smartptr.shared.const/10 is equivalent for our purpose:

效果:构造一个拥有对象p和删除器d的shared_ptr对象.当T不是数组类型时，第一和第二个构造函数使用p启用shared_from_this.第二和第四构造函数应使用a的副本为内部使用分配内存.如果引发异常，则调用d(p).

Effects: Constructs a shared_­ptr object that owns the object p and the deleter d. When T is not an array type, the first and second constructors enable shared_­from_­this with p. The second and fourth constructors shall use a copy of a to allocate memory for internal use. If an exception is thrown, d(p) is called.

因此，如果需要在分配的内存中分配分配器，则使用分配器.根据当前标准和相关缺陷报告，分配不是强制性的，而是由委员会承担的.

So the allocator is used if there is a need to allocate it in allocated memory. Based on the current standard and at relevant defect reports, allocation is not mandatory but assumed by the committee.

• 尽管shared_ptr的接口允许一种实现，其中永远不存在控制块，并且所有shared_ptr和weak_ptr都放在链接列表中，但实际上没有这种实现.另外，例如，假设共享use_count，措词也进行了修改.

• Although the interface of shared_ptr allows an implementation where there is never a control block and all shared_ptr and weak_ptr are put in a linked list, there is no such implementation in practice. Additionally, the wording has been modified assuming, for instance, that the use_count is shared.

删除器仅可移动可构造的.因此，shared_ptr中不可能有多个副本.

The deleter is required to only move constructible. Thus, it is not possible to have several copies in the shared_ptr.

可以想象有一种实现，该实现将删除器放入专门设计的shared_ptr中，并在删除特殊的shared_ptr时将其移动.尽管实现看起来是一致的，但也很奇怪，特别是因为使用计数可能需要一个控制块(使用计数来做相同的事情也许是可能的，但甚至很奇怪).

One can imagine an implementation which puts the deleter in a specially designed shared_ptr and moves it when it the special shared_ptr is deleted. While the implementation seems conformant, it is also strange, especially since a control block may be needed for the use count (it is perhaps possible but even weirder to do the same thing with the use count).

我发现了相关的DR: 545 ， 575 ， 2434 (它承认所有实现使用控制块，似乎暗示多线程约束在某种程度上要求它)，

Relevant DRs I found: 545, 575, 2434 (which acknowledge that all implementations are using a control block and seem to imply that multi-threading constraints somewhat mandate it), 2802 (which requires that the deleter only move constructible and thus prevents implementation where the deleter is copied between several shared_ptr's).

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