Class template can be instantiated without members?

Question

The Wikipedia article says this:

instantiating a class template does not cause its member definitions to be instantiated.

I can't imagine any class in C++ being instantiated, whether from a template or not, where that classes members were not also instantiated?

Answer 1

Many early C++ compilers instantiated all member functions, whether you ever called them or not.

Consider, for example, std::list, which has a sort member function. With a current, properly functioning compiler, you can instantiate list over a type that doesn't support comparison. If you try to use list::sort, it will fail, because you don't support comparison. As long as you don't call sort for that list, it's all fine though, because list<T>::sort won't be instantiated unless you call it.

With those older, poorly functioning compilers, however, trying to create list<T> meant that list<T>::sort was instantiated even though you never used it. The existence of list::sort meant that you needed to implement < for T, just to create a list<T>, even if you never actually used sort on a list of that type at all.

Answer 2

The standard clearly says that (both non-template and template) member methods instantiation should happen only when used.

An excerpt from C++ standard (N3690 - 14.7.1(2) Implicit instantiation)

2 Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.