How can you tell if you have a dynamically typed C++ class library?, C++ FAQ

C++ FAQ / Section 37 / FAQ 37.5

Section 37:

37.1	What is the "STL"?
37.2	Where can I get a copy of "STL"?
37.3	How can I find a `Fred` object in an STL container of `Fred` such as `std::vector<Fred>`?
37.4	Where can I get help on how to use STL?
37.5	How can you tell if you have a dynamically typed C++ class library?
37.6	What is the NIHCL? Where can I get it?
37.7	Where can I ftp the code that accompanies "Numerical Recipes"?
37.8	Why is my executable so large?
37.9	Where can I get tons and tons of more information on C++ class libraries?

[37.5] How can you tell if you have a dynamically typed C++ class library?

Hint #1: when everything is derived from a single root class, usually Object.
Hint #2: when the container classes (List, Stack, Set, etc) are non-templates.
Hint #3: when the container classes (List, Stack, Set, etc) insert/extract elements as pointers to Object. This lets you put an Apple into such a container, but when you get it out, the compiler knows only that it is derived from Object, so you have to use a pointer cast to convert it back to an Apple*; and you'd better pray a lot that it really is an Apple, cause your blood is on your own head.

You can make the pointer cast "safe" by using dynamic_cast, but this dynamic testing is just that: dynamic. This coding style is the essence of dynamic typing in C++. You call a function that says "convert this Object into an Apple or give me NULL if its not an Apple," and you've got dynamic typing: you don't know what will happen until run-time.

When you use templates to implement your containers, the C++ compiler can statically validate 90+% of an application's typing information (the figure "90+%" is apocryphal; some claim they always get 100%, those who need persistence get something less than 100% static type checking). The point is: C++ gets genericity from templates, not from inheritance.