Forward Declarations and Mutually Dependent Classes If the declaration of a class type ends with the word class and a semicolon - that is, if it has the form
type className =
class;
with no ancestor or class members listed after the word class, then it is a forward declaration. A forward declaration
must be resolved by a defining declaration of the same class within the same type declaration section. In other words,
between a forward declaration and its defining declaration, nothing can occur except other type declarations.
Forward declarations allow mutually dependent classes. For example,
type
TFigure = class; // forward declaration
TDrawing = class
Figure: TFigure;
...
end;
TFigure = class // defining declaration
Drawing: TDrawing;
...
end;
Do not confuse forward declarations with complete declarations of types that derive from TObject without declaring
any class members.
type
TFirstClass = class; // this is a forward declaration
TSecondClass = class // this is a complete class declaration
end; //
TThirdClass = class(TObject); // this is a complete class declaration
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