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type
 className = class [abstract | sealed] （ancestorClass）
memberList
 end;

//定义
 type TMemoryStream = class（TCustomMemoryStream）
private
FCapacity: Longint;
procedure SetCapacity（NewCapacity: Longint）;
protected
function Realloc（var NewCapacity: Longint）: Pointer; virtual;
property Capacity: Longint read FCapacity write SetCapacity;
public
destructor Destroy; override;
procedure Clear;
procedure LoadFromStream（Stream: TStream）;
procedure LoadFromFile（const FileName: string）;
procedure SetSize（NewSize: Longint）; override;
function Write（const Buffer; Count: Longint）: Longint; override;
end;
//使用
 var stream: TMemoryStream;
 stream := TMemoryStream.Create;

//继承一个类
type TSomeControl = class（TControl）;
//根类 TObject
type TMyClass = class
 ...
 end;
//等价于
type TMyClass = class（TObject）
 ...
 end;