结构型模式：桥接模式
假设你有一个绘图程序，需要支持绘制多种颜色的多种形状。如果直接将 n n n种形状和 m m m个颜色结合在一起，会导致类的数量急剧增加（ n × m n\times m n×m个）。比如，3种形状（如圆形、矩形和三角形），3种颜色（如红色、蓝色和绿色）就需要9个类：
cpp
#include <iostream>
// 形状类（圆形）
class RedCircle {
public:
void draw() {
std::cout << "Drawing Red Circle\n";
}
};
class BlueCircle {
public:
void draw() {
std::cout << "Drawing Blue Circle\n";
}
};
class GreenCircle {
public:
void draw() {
std::cout << "Drawing Green Circle\n";
}
};
// 形状类（矩形）
class RedRectangle {
public:
void draw() {
std::cout << "Drawing Red Rectangle\n";
}
};
class BlueRectangle {
public:
void draw() {
std::cout << "Drawing Blue Rectangle\n";
}
};
class GreenRectangle {
public:
void draw() {
std::cout << "Drawing Green Rectangle\n";
}
};
// 形状类（三角形）
class RedTriangle {
public:
void draw() {
std::cout << "Drawing Red Triangle\n";
}
};
class BlueTriangle {
public:
void draw() {
std::cout << "Drawing Blue Triangle\n";
}
};
class GreenTriangle {
public:
void draw() {
std::cout << "Drawing Green Triangle\n";
}
};
// 客户端代码
int main() {
// 创建不同颜色的形状组合
RedCircle redCircle;
BlueCircle blueCircle;
GreenCircle greenCircle;
RedRectangle redRectangle;
BlueRectangle blueRectangle;
GreenRectangle greenRectangle;
RedTriangle redTriangle;
BlueTriangle blueTriangle;
GreenTriangle greenTriangle;
// 绘制形状
redCircle.draw(); // 输出: Drawing Red Circle
blueCircle.draw(); // 输出: Drawing Blue Circle
greenCircle.draw(); // 输出: Drawing Green Circle
redRectangle.draw(); // 输出: Drawing Red Rectangle
blueRectangle.draw(); // 输出: Drawing Blue Rectangle
greenRectangle.draw(); // 输出: Drawing Green Rectangle
redTriangle.draw(); // 输出: Drawing Red Triangle
blueTriangle.draw(); // 输出: Drawing Blue Triangle
greenTriangle.draw(); // 输出: Drawing Green Triangle
return 0;
}桥接模式将"高层次的接口或类"与"低层次的接口或类"分离 (...形状有...颜色，形状比颜色高一层)，使得系统具有更好的灵活性和可扩展性。
下面使用桥接模式改写上文的代码：
cpp
#include <iostream>
#include <memory>
// 实现层次：颜色接口
class Color {
public:
virtual void fill() = 0;
virtual ~Color() = default;
};
// 具体颜色类
class Red : public Color {
public:
void fill() override {
std::cout << "Filling with Red color\n";
}
};
class Blue : public Color {
public:
void fill() override {
std::cout << "Filling with Blue color\n";
}
};
class Green : public Color {
public:
void fill() override {
std::cout << "Filling with Green color\n";
}
};
// 抽象类：形状
class Shape {
protected:
std::unique_ptr<Color> color;
public:
Shape(std::unique_ptr<Color> c) : color(std::move(c)) {}
virtual void draw() = 0;
virtual ~Shape() = default;
};
// 具体形状类
class Circle : public Shape {
public:
Circle(std::unique_ptr<Color> c) : Shape(std::move(c)) {}
void draw() override {
color->fill();
std::cout << "Drawing Circle\n";
}
};
class Rectangle : public Shape {
public:
Rectangle(std::unique_ptr<Color> c) : Shape(std::move(c)) {}
void draw() override {
color->fill();
std::cout << "Drawing Rectangle\n";
}
};
class Triangle : public Shape {
public:
Triangle(std::unique_ptr<Color> c) : Shape(std::move(c)) {}
void draw() override {
color->fill();
std::cout << "Drawing Triangle\n";
}
};
// 客户端代码
int main() {
// 创建不同颜色的形状组合
std::unique_ptr<Shape> shapes[9];
shapes[0] = std::make_unique<Circle>(std::make_unique<Red>());
shapes[1] = std::make_unique<Circle>(std::make_unique<Blue>());
shapes[2] = std::make_unique<Circle>(std::make_unique<Green>());
shapes[3] = std::make_unique<Rectangle>(std::make_unique<Red>());
shapes[4] = std::make_unique<Rectangle>(std::make_unique<Blue>());
shapes[5] = std::make_unique<Rectangle>(std::make_unique<Green>());
shapes[6] = std::make_unique<Triangle>(std::make_unique<Red>());
shapes[7] = std::make_unique<Triangle>(std::make_unique<Blue>());
shapes[8] = std::make_unique<Triangle>(std::make_unique<Green>());
// 绘制形状
for (const auto& shape : shapes) {
shape->draw();
}
return 0;
}在没有使用桥接模式的情况下，每种形状和颜色组合都需要一个独立的类，随着形状和颜色的增加，类的数量会迅速膨胀。通过桥接模式，你只需为形状和颜色分别定义类，这大大减少了需要实现的类的数量。