C++ templates that accept only certain types

Question

In Java you can define generic class that accept only types that extends class of your choice, eg:

public class ObservableList {
  ...
         


        

Answer 1

The simple solution, which no one have mentioned yet, is to just ignore the problem. If I try to use an int as a template type in a function template that expects a container class such as vector or list, then I will get a compile error. Crude and simple, but it solves the problem. The compiler will try to use the type you specify, and if that fails, it generates a compile error.

The only problem with that is that the error messages you get are going to be tricky to read. It is nevertheless a very common way to do this. The standard library is full of function or class templates that expect certain behavior from the template type, and do nothing to check that the types used are valid.

If you want nicer error messages (or if you want to catch cases that wouldn't produce a compiler error, but still don't make sense) you can, depending on how complex you want to make it, use either Boost's static assert or the Boost concept_check library.

With an up-to-date compiler you have a built_in static_assert, which could be used instead.