Is std::vector a `user-defined type`?

Question

In 17.6.4.2.1/1 and 17.6.4.2.1/2 of the current draft standard restrictions are placed on specializations injected by users into namespace std.

Answer 1

As users do not define std::vector, and std::vector is not dependent on any type a user defines, std::vector is not a user-defined type.

The logical counter argument is that users do define std::vector. You see std::vector is a class template and as such has no direct representation in binary code.

In a sense it gets it binary representation through the instantiation of a type, so the very action of declaring a std::vector object is what gives "soul" to the template (pardon the phrasing). In a program where noone uses a std::vector this data type does not exist.

On the other hand, following the same argument, std::vector is not a user defined type, it is not even a type, it does not exist; only if we want to (instantiate a type), it will mandate how a structure will be layed out but until then we can only argue about it in terms of structure, design, properties and so on.

Note

The above argument (about templates being not code but ... well templates for code) may seem a bit superficial but draws it's logic, from Mayer's introduction in A. Alexandrescu's book Modern C++ Design. The relative quote there, goes like this :

Eventually, Andrei turned his attention to the development of template-based implementations of popular language idioms and design patterns, especially the GoF[*] patterns. This led to a brief skirmish with the Patterns community, because one of their fundamental tenets is that patterns cannot be represented in code. Once it became clear that Andrei was automating the generation of pattern implementations rather than trying to encode patterns themselves, that objection was removed, and I was pleased to see Andrei and one of the GoF (John Vlissides) collaborate on two columns in the C++ Report focusing on Andrei's work.