为什么 std::vector 在类定义中使用不完整的类型？

Question

The following question came up:

The c++ standard seems to say, that std::vector requires a complete type to work. (See https://en.cppreference.com/w/cpp/container/vector ) Then, why does the following code still compile?

#include <vector>

struct parent;

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

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

This seems counterintuitive. If std::vector requires a complete type, then std::vector<parent> should not compile because only its forward declaration is known inside the class definition of child .

• Is this behaviour something special about class definitions?
• Did I get it wrong, and std::vector does not require a complete type?
• Or, is this just a fluke? In that technically this isn't allowed, but it works for all implementations anyways...

EDIT

There seems to be a difference between c++11 and c++17. I would like to understand the c++11 version.

Answer 1

Standard says (draft N3690; this is post C++11, pre C++14):

[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.

Given that standard places no requirements, and effects are undefined (as far as I can tell, this is same as undefined behaviour), there is no expectation for the instantiation to "not work" any more than there is expectation for it to (appear to) "work".

---

Since C++17, the requirement was relaxed and std::vector does not require the value type to be complete, if used with appropriate allocator (the default allocator is appropriate). (This freedom does not extend to using all member functions; they have additional requirements).

Standard quote (current draft):

[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]).