C++: Polymorphic class template

Question

Consider a class Calendar that stores a bunch of Date objects. The calendar is designed to hold a collection of any type of objects that inherit from Date. I thought the best way to do it is to have a class template such as ``` template class Calendar{ ... } ``` But it struck me that D can now in fact be any class. My question is now, how can I make sure that D is a subclass of the date object? I know how to do this is Java, but I'm still unfamiliar with the C++ syntax. The problem is very much similar to how some collections can only take a template variables that implement Comparable. The header would then look something like ``` public class Calendar{ ... } ``` --------------------EDIT: ------------------------------------------ The template argument defines which actual day the calendar refers to. Different date types refer to the same day in different formats. For instance, if I make a `Calendar` it will be able to take dates in another `Dat…

Accepted Answer

I know how to do this is Java, but I'm still unfamiliar with the C++ syntax. The problem is very much similar to how some collections can only take a template variables that implement Comparable. The header would then look something like

```
public class Calendar<D extends Date>{
     ...
}
```

True, it is the same problem, and in C++, it is usually solved by ignoring it. Why do we need to enforce that the object must implement `IComparable`? In Java, it's necessary because of its anemic type system. Without this constraint, we'd be unable to compare objects.

In C++, the rules are different. Containers simply try to compare the objects they store, and if the type doesn't support it, you get a compile error. No interfaces or inheritance is required.

And you'd typically do the same in your `Calendar` class. Simply don't enforce the "must subclass form `Date` constraint.

Instead, specify the members the type must expose, and what, if any, semantics should be expected from them.

For example, if your Calendar attempts to do the following operations, for date objects `d0` and `d1`:

```
d0.getDay();
d0.getTime();
Time t = d0 - d1;
```

Then those are the operations that should be supported. Any class which supports these operations is a valid Date class, even if it doesn't subclass anything.

C++: Polymorphic class template

Solution

Problem