Original
Class-reference types
A class-reference type, sometimes
called a metaclass, is denoted by a construction of the form
class of type
where type is any class type. The
identifier type itself denotes a value whose type is class of type.
If type1 is an ancestor of type2, then class of type2 is
assignment-compatible with class of type1. Thus
type
TClass = class of TObject;
var
AnyObj: TClass;
declares a variable called AnyObj
that can hold a reference to any class. (The definition of a class-reference
type cannot occur directly in a variable declaration or parameter list.) You
can assign the value nil to a variable of any class-reference type.
To see how
class-reference types are used, look at the declaration of the constructor for TCollection
(in the Classes unit):
type
TCollectionItemClass = class of TCollectionItem;
...
constructor
Create(ItemClass: TCollectionItemClass);
This declaration says that to
create a TCollection instance object, you must pass to the constructor
the name of a class descending from TCollectionItem.
Class-reference types are useful when you want to invoke a class method or virtual constructor on a class or object whose actual type is unknown at compile time.
Constructors and class references
See also
译文
类引用类型
类引用类型有时叫做元类（metaclass），表示类引用类型的结构具有如下形式
class of type
这里的type是任何类类型。标识符type自身表示一个值，它的类型是type类。如果type1是type2的祖先，那么type2的类与type1的类是赋值兼容的。因此，
type
TClass = class of TObject;
var
AnyObj: TClass;
这里声明一个叫做AnyObj的变量，用于可以保存对任何类的引用。（类引用类型的定义不能直接出现在变量声明或参数列表声明中。）可以把值nil赋给任何类引用类型的变量。
要明白类引用类型是如何被使用的，可以查看TCollection（Classes单元中）的构造器声明：
type
TCollectionItemClass = class of TCollectionItem;
...
constructor
Create(ItemClass: TCollectionItemClass);
这里的声明表明，要创建TCollection的实例对象，就必需向构造器传递TCollectionItem类或自该类派生的类的名称。
编译时，想要调用类或对象的类方法或者虚拟的构造器，而不知道其实际类型时，类引用类型是有用的。