C++如何应用SOLID设计原则？（代码示例）

// ❌ 违反SRP：Order既处理业务逻辑，又负责写日志
class Order {
public:
    void process() { /* ... */ }
    void logToDatabase() { /* ... */ }  // 职责混杂
};
<p>// ✅ 符合SRP：职责拆分，Order专注业务，Logger专注输出
class Order {
std::string id_;
public:
void process() { /<em> 核心流程，不碰IO </em>/ }
};</p><p>class OrderLogger {
public:
void log(const Order& order) {
std::cout << "Logging order: " << order.id_ << "\n";
}
};

// ✅ 支持添加新支付方式，无需动PaymentProcessor原有逻辑
class PaymentStrategy {
public:
    virtual ~PaymentStrategy() = default;
    virtual void pay(double amount) const = 0;
};
<p>class CreditCardStrategy : public PaymentStrategy {
void pay(double amount) const override {
std::cout << "Charging $" << amount << " to credit card\n";
}
};</p><p>class PayPalStrategy : public PaymentStrategy {
void pay(double amount) const override {
std::cout << "Paying $" << amount << " via PayPal\n";
}
};</p><p>class PaymentProcessor {
std::unique<em>ptr<PaymentStrategy> strategy</em>;
public:
void setStrategy(std::unique<em>ptr<PaymentStrategy> s) {
strategy</em> = std::move(s);
}
void execute(double amount) {
if (strategy<em>) strategy</em>->pay(amount);
}
};

// ❌ 违反LSP：Rectangle和Square语义冲突
class Rectangle {
protected:
    double width_, height_;
public:
    virtual void setWidth(double w) { width_ = w; }
    virtual void setHeight(double h) { height_ = h; }
    double area() const { return width_ * height_; }
};
<p>class Square : public Rectangle {
public:
void setWidth(double w) override { width<em> = height</em> = w; }
void setHeight(double h) override { width<em> = height</em> = h; }
// 问题：用户调用setHeight后，width也变了，area行为不可预测
};</p><p>// ✅ 更安全做法：不用继承，用组合 + 接口
class Shape {
public:
virtual double area() const = 0;
virtual ~Shape() = default;
};</p><p>class Rectangle : public Shape {
double w<em>, h</em>;
public:
Rectangle(double w, double h) : w<em>(w), h</em>(h) {}
double area() const override { return w<em> * h</em>; }
};</p><p>class Square : public Shape {
double side<em>;
public:
Square(double s) : side</em>(s) {}
double area() const override { return side<em> * side</em>; }
};

// ❌ 大而全的接口，PrinterUser必须实现所有，哪怕只打印
class Printer {
public:
    virtual void print() = 0;
    virtual void scan() = 0;
    virtual void fax() = 0;
};
<p>// ✅ 拆成三个独立接口，按需实现
class Printable { public: virtual void print() = 0; };
class Scannable { public: virtual void scan() = 0; };
class Faxable   { public: virtual void fax() = 0; };</p><p>class SimplePrinter : public Printable {
void print() override { std::cout << "Printing...\n"; }
};</p><p>class MultiFunctionDevice : public Printable, public Scannable, public Faxable {
void print() override { /<em> ... </em>/ }
void scan()  override { /<em> ... </em>/ }
void fax()   override { /<em> ... </em>/ }
};

// ❌ 高层直接new具体类，紧耦合
class UserService {
    Database db_; // 直接依赖具体类
public:
    bool saveUser(const User& u) { return db_.save(u); }
};
<p>// ✅ 依赖抽象，运行时注入具体实现
class DatabaseInterface {
public:
virtual ~DatabaseInterface() = default;
virtual bool save(const User& u) = 0;
};</p><p>class UserService {
std::unique<em>ptr<DatabaseInterface> db</em>;
public:
explicit UserService(std::unique<em>ptr<DatabaseInterface> db)
: db</em>(std::move(db)) {}</p><pre class="brush:php;toolbar:false;">bool saveUser(const User& u) { return db_->save(u); }