c++ - 为什么 std::vector 使用类定义中的不完整类型？

Question

下面的问题出现了:

C++ 标准似乎说，std::vector需要一个完整的类型才能工作。 (参见 https://en.cppreference.com/w/cpp/container/vector)那么，为什么下面的代码仍然可以编译？

#include <vector>

struct parent;

struct child
{
    std::vector<parent> parents; //parent is incomplete here!
};

struct parent
{
    std::vector<child> children;
};

这似乎违反直觉。如果std::vector需要一个完整的类型，然后 std::vector<parent>不应编译，因为在 child 的类定义中只有它的前向声明是已知的.

• 这种行为是否与类定义有关？
• 我是不是弄错了，std::vector不需要完整类型？
• 或者，这只是侥幸？从技术上讲，这是不允许的，但无论如何它都适用于所有实现......

编辑

c++11 和 c++17 之间似乎有区别。我想了解c++11版本。

Answer 1

标准说(草案 N3690；这是 C++11 之后，C++14 之前):

[res.on.functions]

1 In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++standard library depends on components supplied by a C++program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

2 In particular, the effects are undefined in the following cases:

— if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.

鉴于标准没有要求，并且效果是未定义的(据我所知，这与未定义的行为相同)，没有期望实例化“不起作用”比期望它更多去(看起来)“工作”。

---

从 C++17 开始，要求放宽了，如果与适当的分配器(默认分配器是合适的)。 (这种自由并不扩展到使用所有成员函数；它们有额外的要求)。

标准报价(当前草案):

[vector.overview]

An incomplete type T may be used when instantiating vector if the allocator meets the allocator completeness requirements. T shall be complete before any member of the resulting specialization of vector is referenced.

[allocator.requirements.completeness]

If X is an allocator class for type T, X additionally meets the allocator completeness requirements if, whether or not T is a complete type:

• X is a complete type, and
• all the member types of allocator_­traits other than value_­type are complete types.

[default.allocator]

All specializations of the default allocator meet the allocator completeness requirements ([allocator.requirements.completeness]).