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07: Objects - 215
internal call
this is a method of a nested/nested class
constant is 12
How would you benefit from using a nested class in the Object Pascal language? The
concept is commonly used in Java to implement event handler delegates and makes
sense in C# where you cannot hide a class inside a unit. In Object Pascal nested
classes are the only way you can have a field of the type of another private class (or
inner class) without adding it to the global name space and making it globally visible.
If the internal class is used only by a method, you can achieve the same effect by
declaring the class within the implementation portion of the unit. But if the inner
class is referenced in the interface section of the unit (for example because it is used
for a field or a parameter), it must be declared in the same interface section and will
end up being visible. The trick of declaring such a field of a generic or base type and
then casting it to the specific (private) type is much less clean than using a nested
class.
note
In chapter 10 there is a practical example in which nested classes come in handy, namely imple menting a custom iterator for a for in loop.
Marco Cantù, Object Pascal Handbook