我的Design Pattern之旅[7]：使用泛型改進Adapter Pattern(OO)(3)

在.Net語言內，C++/CLI是唯一得天獨厚有兩種泛型的語言，C++/CLI可以用ISO C++既有的template，也可以用CLI的Generics。有了template，C++/CLI也可以實現這個技巧。

69行

template<typename T>
ref class DrawAdapter : public IDrawStrategy, public T {
public:
　virtual void draw();
};

80行

int main() {
　Grapher^ grapher = gcnew Grapher(gcnew DrawAdapter<Triangle>);
　grapher->drawShape();

　grapher->setShape(gcnew DrawAdapter<Circle>);
　grapher->drawShape();

　grapher->setShape(gcnew DrawAdapter<Square>);
　grapher->drawShape();
}

C++/CLI也可以在ClIEnt動態改變泛型參數來改善Class Adapter。

看到這裡，你或許會問，C#可以嗎?既然C# 2.0也有泛型，答案很遺憾，目前C# 2.0不行，C# 3.0我還沒試，或許有興趣的朋友可以試看看。

C#

/**//*
(C) OOMusou 2007 http://oomusou.cnblogs.com

Filename　　: DP_AdpaterPattern_Strategy_ClassByTemplate_Error.cs
Compiler　　: Visual Studio 2005 / C# 2.0
Description : Demo how to use Strategy Pattern with Adpater Pattern (Class Adapter) By Template
Release　　 : 07/11/2007 1.0
*/
using System;

interface IDrawStrategy {
　void draw();
}

class Grapher {
　protected IDrawStrategy _drawStrategy;

　public Grapher() { }
　public Grapher(IDrawStrategy drawStrategy) {
　　_drawStrategy = drawStrategy;
　}

　public void drawShape() {
　　if (_drawStrategy != null)
　　　_drawStrategy.draw();
　}

　public void setShape(IDrawStrategy drawStrategy) {
　　_drawStrategy = drawStrategy;
　}
}

interface IPaint {
　void paint();
}

class Triangle : IPaint {
　public void paint() {
　　Console.WriteLine("Draw Triangle");
　}
}

class Circle : IPaint {
　public void paint() {
　　Console.WriteLine("Draw Circle");
　}
};

class Square : IPaint {
　public void paint() {
　　Console.WriteLine("Draw Square");
　}
}

// Error : Cannot derive from 'T' because it is a type parameter
class DrawAdapter<T> : IDrawStrategy, T {
　public void draw() {
　　paint();
　}
}

class ClIEnt {
　static void Main() {
　　Grapher grapher = new Grapher(new DrawAdapter<Triangle>());
　　grapher.drawShape();

　　grapher.setShape(new DrawAdapter<Circle>());
　　grapher.drawShape();

　　grapher.setShape(new DrawAdapter<Square>());
　　grapher.drawShape();
　}
}

以上C#無法compile成功，錯在55行

// Error : Cannot derive from 'T' because it is a type parameter
class DrawAdapter<T> : IDrawStrategy, T {
　public void draw() {
　　paint();
　}
}

錯誤訊息為

Error : Cannot derive from 'T' because it is a type parameter