Lambda the Ultimate

I don't know how Ada handles parametric polymorphism. C++ uses templates from which the compiler generates specialized code. So a List<int> generates one class and a List<double> generates a different class. It's almost exactly as if you had hand written a specialized List_int and a specialized List_double. Each such variation of List is statically compiled and linked in C++.

Sticking with the List example, many other languages with parametric polymorphism (e.g. Java) will use boxing to handle simple "primitives" like int and double, so as far as memory layout is concerned there would only be one List construct: List of references/pointers. The type system will then ensure that different types of List are treated appropriately. Depending on the language, the runtime may also treat them differently by tagging each instance of List with its parameter type. Boxing and unboxing are generally handled automatically by the compiler.

Edit: I remembered belatedly a third option. In some Haskell and Scheme implementations (and probably others) a few bits out of each reference/pointer are designated as a tag to indicate if the value is in fact a pointer or if it's a directly captured primitive like an int. This cuts down on boxing overhead at the price of a few bits per reference. Otherwise it behaves a lot like the previous option.

Edit: fixed angle bracket issues