Forward declaration of objects with STL containers

Question

Consider the following code snippet, where the first line serves only as forward declaration

 class A;

followed by defining new class

Answer 1

[As supplemental instruction to Barry's answer]

According to the standard (C++17), only std::vector, std::list and std::forward_list could be used with incomplete type when instantiating.

§23.3.11.1/3 Class template vector overview [vector.overview]:

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

§23.3.9.1/4 Class template forward_list overview [forwardlist.overview]:

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

§23.3.10.1/3 Class template list overview [list.overview]:

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