策略模式

定义一系列算法，把它们一个个封装起来，并且使它们可互相替换（变化）。该模式使得算法可独立于使用它的客户程序（稳定）而变化（扩展，子类化）。

阶段一

enum TaxBase{
  CN_TAX,
  US_TAX,
  DE_TAX,
};

class SalesOrder{
public:
  double calculateTax()
  {
    //...
    
    if(tax_base_==TaxBase::CN_TAX)
    {
      //...
    }
    else if(tax_base_==TaxBase::US_TAX)
    {
      //...
    }
    else if(tax_base_==TaxBase::DE_TAX)
    {
      //...
    }
    
    //...
  }
private:
  TaxBase tax_base_;
}

阶段二

class TaxStrategy
{
public:
  TaxSalesStrategy(){ }
  virtual ~TaxSalesStrategy(){ }
  
  virtual double calculate(Context& context) = 0;
}

class CNTaxStrategy
{
public:
  virtual double calculate(Context& context) override
  {
    //...
  }
}

class USTaxStrategy
{
public:
  virtual double calculate(Context& context)	override
  {
    //...
  }
}

class DETaxStrategy
{
public:
  virtual double calculate(Context& context) override
  {
    //...
  }
}

类图

classDiagram class Context{ +ContextInterface() } class Strategy{ +algorithmInterface() } class ConcreteStrategyA{ +algorithmInterface() } class ConcreteStrategyB{ +algorithmInterface() } class ConcreteStrategyC{ +algorithmInterface() } Context o-- Strategy : strategy Strategy <|-- ConcreteStrategyA Strategy <|-- ConcreteStrategyB Strategy <|-- ConcreteStrategyC