我的Design Pattern之旅[3]：使用template改進Strategy Pattern(OO)(3)

這樣的設計看似完美，但IShape和Grapher相依程度太高，若將來有個Painter class，和Grapher完全不同，沒有任何繼承或多型的關系，但想重復使用IShape interface的strategy，這樣的設計就無法讓Painter使用了。若我們能讓IShape interface的draw()不再只限定Grapher型別，改用template，就能讓將來所有型別都能使用IShape interface。

template <typename T>
class IShape {
public:
　virtual void draw(T &grapher) const = 0;
};

我們用泛型T取代了Grapher，任何型別都可傳進IShape::draw()。

完整程式碼如下

1/**//*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename　　: DP_StrategyPattern3_template_this.cpp
5Compiler　　: Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6Description : Demo how to use Strategy Pattern with *this by template
7Release　　 : 04/03/2007 1.0
8*/
9#include <iOStream>
10#include <string>
11
12using namespace std;
13
14class Grapher;
15
16template <typename T>
17class IShape {
18public:
19　virtual void draw(T &grapher) const = 0;
20};
21
22class Grapher {
23private:
24　IShape<Grapher> *shape;
25　string text;
26
27public:
28　Grapher() {}
29　Grapher(IShape<Grapher> *shape, const char* text = "Hello Shape!!") : shape(shape), text(string(text)) {}
30
31public:
32　void drawShape() {
33　　this->shape->draw(*this);
34　}
35
36　void setShape(IShape<Grapher> *shape, const char* text = "Hello Shape!!") {
37　　this->text = text;
38　　this->shape = shape;
39　}
40
41　string getText() const {
42　　return this->text;
43　}
44};
45
46template <typename T>
47class Triangle : public IShape<T> {
48public:
49　void draw(T &grapher) const {
50　　cout << "Draw " << grapher.getText() << " in Triangle" << endl;
51　}
52};
53
54template <typename T>
55class Circle : public IShape<T> {
56public:
57　void draw(T &grapher) const {
58　　cout << "Draw " << grapher.getText() << " in Circle" << endl;
59　}
60};
61
62template <typename T>
63class Square : public IShape<T> {
64public:
65　void draw(T &grapher) const {
66　　cout << "Draw " << grapher.getText() << " in Square" << endl;
67　}
68};
69
70
71int main() {
72　Grapher theGrapher(&Square<Grapher>());
73　theGrapher.drawShape();
74
75　theGrapher.setShape(&Circle<Grapher>(), "Hello C++!!");
76　theGrapher.drawShape();
77}

執行結果

Draw Hello Shape!! in Square
Draw Hello C++!! in Circle

Conclusion

泛型的應用相當廣，在此范例僅僅是泛型的小小應用，在OOP世界使用strategy pattern，常將*this傳給strategy，若搭配GP可讓strategy pattern的resuse程度更高。