Predefined Generic Interfaces

Now that we have explored how to define generic interfaces and combine them with the use of generic and specific classes, we can get back to having a second look to the Generics.Default unit. This unit defines two generic comparison interfaces:

  • IComparer<T> has a Compare method
  • IEqualityComparer<T> has Equals and GetHashCode methods

These classes are implemented by some generic and specific classes, listed below (with no implementation details): type

TComparer<T> = class(TInterfacedObject, IComparer<T>) TEqualityComparer<T> = class(

TInterfacedObject, IEqualityComparer<T>) TCustomComparer<T> = class(TSingletonImplementation,

IComparer<T>, IEqualityComparer<T>) TStringComparer = class(TCustomComparer<string>)

In the listing above you can see that the base class used by the generic implementations of the interfaces is either the classic reference-counted TInterfacedObject class or the new TSingletonImplementation class. This is an oddly named87 class that provides a basic implementation of IInterface with no reference counting.

As we have already seen in the "Sorting a TList<T>" section earlier in this chapter, these comparison classes are used by the generic containers. To

87 The term singleton is generally used to define a class of which you can create only one instance, and not one with no reference counting. I consider this quite a misnomer.

make things more complicated, though, the Generics.Default unit relies quite heavily on anonymous methods, so you should probably look at it only after reading the next chapter.

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