Original
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
See also
译文
向前声明和相互依赖的类
如果一个类类型的声明以保留字class和紧随其后的分号结束，即具有如下形式
type className = class;
这里没有任何祖先或类成员列表于保留字class之后，那么，这是一个向前声明（forward declaration）。一个向前声明必需被同一个类的定义声明（defining declaration）解释，并且二者位于相同的类型声明节。换句话说，在向前声明和它的定义声明之间，除了其他的类型声明之外，不能出现任何其他内容。
向前声明允许实现互相依赖的类。例如，
type
TFigure = class; //向前声明
TDrawing = class
Figure: TFigure;
...
end;
TFigure = class //定义声明
Drawing: TDrawing;
...
end;
不要混淆向前声明和起源于TObject类而没有声明任何类成员的类类型的完整声明。例如
type
TFirstClass = class; //这是一个向前声明
TSecondClass = class //这是一个完整的类声明
end; //
TThirdClass = class(TObject); //这是一个完整的类声明