《GOF设计模式》—原型(Prototype)—Delphi源码示例：浅拷贝和深拷贝



示例：浅拷贝和深拷贝

说明：

Prototype模式最困难的部分在于正确实现Clone操作。例如，当所考虑的类已经存在时就难以新增Clone操作。当类内部包括一些不支持拷贝或有循环引用的对象时，实现克隆可能会很困难。

大多数语言都对克隆对象提供了一些支持。例如，Smalltalk提供了一个copy的实现，它被所有Object的子类所继承。C++提供了一个拷贝构造器。但这些措施并不能解决“浅拷贝和深拷贝”问题。也就是说，克隆一个对象是依次克隆它的实例变量呢，还是由克隆对象和原对象共享这些变量？

浅拷贝简单并且通常也足够了。C++中的缺省拷贝构造器实现按成员拷贝，这意味着在拷贝的和原来的对象之间是共享指针的。但克隆一个结构复杂的原型通常需要深拷贝（Deepcopy），因为复制对象和原对象必须相互独立。因此你必须保证克隆对象的构件也是对原型的构件的克隆。克隆迫使你决定如果所有东西都被共享了该怎么办。

如果系统中的对象提供了Save和Load操作，那么你只需通过保存对象和立刻载入对象，就可以为Clone操作提供一个缺省实现。Save操作将该对象保存在内存缓冲区中，而Load则通过从该缓冲区中重构这个对象来创建一个复本。

代码：

unit uPrototypeClone2;

 

interface

 

uses

   Classes;

 

type

   TStateObject = class

   private

       FState: string;

   public

       //---

       property State: string read FState write FState;

   end;

 

   TBaseObject = class

   private

       FState: string;

   public

       function Clone: TBaseObject; virtual; abstract;

       function GetAllState: string; virtual;

       //---

       property State: string read FState write FState;

   end;

 

   TObjectA = class(TBaseObject)

   public

       function Clone: TBaseObject; override; //--浅拷贝

   end;

 

   TObjectB = class(TBaseObject)

   private

       FMyState: TStateObject;

       procedure SetMyState(const Value: TStateObject);

   public

       constructor Create;

       destructor Destroy; override;

       //---

       function Clone: TBaseObject; override; //--深拷贝

       function GetAllState: string; override;

       //---

       property MyState: TStateObject read FMyState write SetMyState;

   end;

 

   TObjectC = class(TBaseObject)

   private

       FMyState: TBaseObject;

       procedure SetMyState(const Value: TBaseObject);

   public

       constructor Create;

       destructor Destroy; override;

       //---

       function Clone: TBaseObject; override; //--深拷贝

       function GetAllState: string; override;

       //---

       property MyState: TBaseObject read FMyState write SetMyState;

   end;

 

   TClient = class

   private

       FPrototype: TBaseObject;

   public

       constructor Create(Prototype: TBaseObject);

       destructor Destroy; override;

       //---

       function CreateObject: TBaseObject;

   end;

 

implementation

 

function TBaseObject.GetAllState: string;

begin

   Result := FState;

end;

 

function TObjectA.Clone: TBaseObject;

var

   AObject: TObjectA;

begin

   AObject := TObjectA.Create;

   AObject.State := self.State;

   //---

   Result := AObject;

end;

 

constructor TObjectB.Create;

begin

   inherited;

   //---

   FMyState := nil;

end;

 

destructor TObjectB.Destroy;

begin

   if FMyState <> nil then

       FMyState.Free;

   //---

   inherited;

end;

 

function TObjectB.Clone: TBaseObject;

var

   AObject: TObjectB;

begin

   AObject := TObjectB.Create;

   AObject.State := self.State;

   if self.MyState <> nil then

   begin

       AObject.MyState := TStateObject.Create;

       AObject.MyState.State := self.MyState.State;

   end;

   //---

   Result := AObject;

end;

 

function TObjectB.GetAllState: string;

begin

   if FMyState <> nil then

       Result := FState + FMyState.State

   else

       Result := FState;

end;

 

procedure TObjectB.SetMyState(const Value: TStateObject);

begin

   if assigned(FMyState) then

       FMyState.Free;

   FMyState := Value;

end;

 

constructor TObjectC.Create;

begin

   inherited;

   //---

   FMyState := nil;

end;

 

destructor TObjectC.Destroy;

begin

   if assigned(FMyState) then

       FMyState.Free;

   //---

   inherited;

end;

 

function TObjectC.Clone: TBaseObject;

var

   AObject: TObjectC;

begin

   AObject := TObjectC.Create;

   AObject.State := self.State;

   if self.MyState <> nil then

       AObject.MyState := self.MyState.Clone;

   //---

   Result := AObject;

end;

 

function TObjectC.GetAllState: string;

begin

   if assigned(FMyState) then

       Result := FState + FMyState.GetAllState

   else

       Result := FState;

end;

 

procedure TObjectC.SetMyState(const Value: TBaseObject);

begin

   if assigned(FMyState) then

       FMyState.Free;

   FMyState := Value;

end;

 

constructor TClient.Create(Prototype: TBaseObject);

begin

   FPrototype := Prototype;

end;

 

destructor TClient.Destroy;

begin

   FPrototype.Free;

   //---

   inherited;

end;

 

function TClient.CreateObject: TBaseObject;

begin

   Result := FPrototype.Clone;

end;

 

end.

 

 

procedure TForm1.Button1Click(Sender: TObject);

var

   APrototype,AObject: TBaseObject;

   AClient: TClient;

begin

   APrototype := TObjectA.Create;

   APrototype.State := 'Prototype A';

   //---

   AClient := TClient.Create(APrototype);

   try

       AObject := AClient.CreateObject;

       showmessage(AObject.GetAllState);

       AObject.Free;

   finally

       AClient.Free;

   end;

end;

 

procedure TForm1.Button2Click(Sender: TObject);

var

   APrototype: TObjectB;

   AObject: TBaseObject;

   AClient: TClient;

begin

   APrototype := TObjectB.Create;

   with APrototype do

   begin

       State := 'Object B';

       MyState := TStateObject.Create;

       MyState.State := ' 123';

   end;

   //---

   AClient := TClient.Create(APrototype);

   try

       AObject := AClient.CreateObject;

       showmessage(AObject.GetAllState);

       AObject.Free;

   finally

       AClient.Free;

   end;

end;

 

procedure TForm1.Button3Click(Sender: TObject);

var

   APrototype: TObjectC;

   AObject: TBaseObject;

   AClient: TClient;

begin

   APrototype := TObjectC.Create;

   with APrototype do

   begin

       APrototype.State := 'Object C';

       APrototype.MyState := TObjectA.Create;

       APrototype.MyState.State := ' 123';

   end;

   //---

   AClient := TClient.Create(APrototype);

   try

       AObject := AClient.CreateObject;

       try

           showmessage(AObject.GetAllState);

           //---

           with TObjectC(AObject) do

           begin

               if (MyState = nil) or (MyState = APrototype.MyState) then

                   showmessage('浅拷贝 AObject.MyState为引用')

               else

                   showmessage('深拷贝 AObject.MyState为新对象');

           end;

       finally

           AObject.Free;

       end;

   finally

       AClient.Free;

   end;

end;