non-dummy-type template parameters

(Note this topic is not related to template Partial specialization)

Majority of class templates in practice have a dummy type (or multiple, but let's stay focused) to signify an Unknown-type. For example, the STL vector can be "Instantiated" in memory to become a real CLASS when given a real type like "Account". The real type replaces the dummy type. [[C++ primer]] is the first book I read that mentions a very powerful type of class template. I call it NDTP...

Background -- What appears inside angle brackets after the keyword "template" is always a list of "tokens" or "thingies". In most cases in practice, each thingy is a "typename T" or "class C".

Now, the thingy can also be something like "int size". This is known as a non-type template parameter. I call it a non-dummy-type parameter, or NDT parameter. a NDT is not a dummy type, but a real type with a variable name. NDT declaration syntax is like a regular function parameter. NDT represents a dummy type pinned to a real type.  But how is NDT used when the push comes to the shove i.e. template instantiation?

Remember a real type argument like "AccountAllocator" in STL vector is a piece of information at the class level, not Instance level. When the vector is instantiated in memory to become a real/concrete Class, the real class uses AccountAllocator Class. In the same vein, the NDT is at class-level, not class-instance level nor at template level. That implies the "int size" value of 55 is a constant for the class, across all class instances.

In other words, when we convert a GENERIC template matrix_double into a real class, the "int size" template parameter (NDT parameter) is replaced by a value like 55, and treated as a class-level static constant. If we construct 9999 instances of the concrete matrix_double class, all of them share the same size=55.

A class template can have all its "tokens" be dummy types (standard practice), or all of them NDT (P861 [[c++primer]]), or a mixture.

See other posts about When the NDT is a func ptr.