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
};
// ✅ 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() { / 核心流程，不碰IO / }
};
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;
};
class CreditCardStrategy : public PaymentStrategy {
void pay(double amount) const override {
std::cout << "Charging $" << amount << " to credit card\n";
}
};
class PayPalStrategy : public PaymentStrategy {
void pay(double amount) const override {
std::cout << "Paying $" << amount << " via PayPal\n";
}
};
class PaymentProcessor {
std::uniqueptr strategy;
public:
void setStrategy(std::uniqueptr s) {
strategy = std::move(s);
}
void execute(double amount) {
if (strategy) strategy->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_; }
};
class Square : public Rectangle {
public:
void setWidth(double w) override { width = height = w; }
void setHeight(double h) override { width = height = h; }
// 问题：用户调用setHeight后，width也变了，area行为不可预测
};
// ✅ 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;
};
class Rectangle : public Shape {
double w, h;
public:
Rectangle(double w, double h) : w(w), h(h) {}
double area() const override { return w * h; }
};
class Square : public Shape {
double side;
public:
Square(double s) : side(s) {}
double area() const override { return side * side; }
};

// ❌ 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;
};
// ✅ 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; };
class SimplePrinter : public Printable {
void print() override { std::cout << "Printing...\n"; }
};
class MultiFunctionDevice : public Printable, public Scannable, public Faxable {
void print() override { / ... / }
void scan()  override { / ... / }
void fax()   override { / ... / }
};