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

There is no keyword for such type checks, but you can put some code in that will at least fail in an orderly fashion:

(1) If you want a function template to only accept parameters of a certain base class X, assign it to a X reference in your function. (2) If you want to accept functions but not primitives or vice versa, or you want to filter classes in other ways, call a (empty) template helper function within your function that's only defined for the classes you want to accept.

You can use (1) and (2) also in member functions of a class to force these type checks on the entire class.

You can probably put it into some smart Macro to ease your pain. :)

Answer 2

An equivalent that only accepts types T derived from type List looks like

template<typename T, 
         typename std::enable_if<std::is_base_of<List, T>::value>::type* = nullptr>
class ObservableList
{
    // ...
};