How come I can use a forward-declared class in a std::vector?

Question

I thought that you could only create a reference or pointer member to a forward-declared class. However, I was surprised to discover this works:

#include <vector>

struct Donkey;

struct Cage
{
    std::vector<Donkey> donkeys;
};

struct Donkey
{
};

int main()
{
    Cage c;
}

http://ideone.com/EP0zKR

How come std::vector can be defined with a forward-declared class? Is this standard?

Answer 1

Actually, you can't.

Just because your program compiles (which is down to facts of the underlying implementation) does not mean it is valid.

There are times other than declaring a T* or a T& at which you may use a forward declaration ; it's just that this is not one of them .

Answer 2

It's undefined behavior.

[C++14/§17.6.4.8] 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.

[C++14/§17.6.4.8] 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.

Furthermore, an incomplete type doesn't meet the allocator requirements:

[C++14/§17.6.3.5] 9 An allocator may constrain the types on which it can be instantiated and the arguments for which its construct member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to construct may fail to instantiate.

Because your class has a definition by the time the vector is instantiated, it's fine. Be on the lookout for this proposal which will change allocator requirements, N4056 Minimal incomplete type support for standard containers