修正算术类中的循环依赖

问题描述

我有一组类实现奇怪的循环模板模式。然而，诀窍是基类需要返回子类的实例。这里有一个例子：

  template< typename SubType> 
 class ArithmeticBase 
 {
 public：
 template< typename OtherType> 
 const加法运算符+（const OtherType& other）
 {return Addition（get_subclass（），other）;} 
 
 // ... 
 //用于减法，乘法，除法，... 
 
 private：
 const SubType& get_subclass（）const 
 {return * static_cast< const SubType *> } 
}; 
 
 template< typename OperatorType1，typename OperatorType2> 
类加法：ArithmeticBase<加法< OperatorType1，OperatorType2>> 
 {
 public：
添加（const OperatorType1& op1，const OperatorType2& op2）
：op1（op1）
，op2（op2）
 {} 
 
 private：
 const OperatorType1& op1; 
 const OperatorType2& op2; 
}; 
 
 // ... 
 //减法，乘法，除法等其他类
  

编译失败，因为添加类未在 ArithmeticBase class：

  arithmetic.cpp：6：8：error：unknown type name'Addition'
 const加法如何解决这个问题？
 
  p> 
 
解决方案
您可以在基类之前转发添加。
 模板< typename OperatorType1，typename OperatorType2> 
类加法; 
 template< typename SubType> 
 class ArithmeticBase 
 {
 ... 
}; 
  
这允许编译器知道有一个类型添加在定义之前存在。
 
I have a set of classes implementing the curiously recurring template pattern. However, the trick is that the base class needs to return instances of the subclasses. Here's an example:
template <typename SubType>
class ArithmeticBase
{
public:
    template <typename OtherType>
    const Addition operator+(const OtherType &other)
        {return Addition(get_subclass(), other);}

    // ...
    // Operators for subtraction, multiplication, division, ...

private:
    const SubType &get_subclass() const
        {return *static_cast<const SubType*>(this);}
};

template <typename OperatorType1, typename OperatorType2>
class Addition : ArithmeticBase<Addition<OperatorType1, OperatorType2>>
{
public:
    Addition(const OperatorType1 &op1, const OperatorType2 &op2)
        : op1(op1)
        , op2(op2)
    {}

private:
    const OperatorType1 &op1;
    const OperatorType2 &op2;
};

// ...
// Additional classes for subtraction, multiplication, division, ...
Compiling this fails because the Addition class is not defined before it's used in the ArithmeticBase class:
arithmetic.cpp:6:8: error: unknown type name 'Addition'
        const Addition operator+(const OtherType &other)
              ^
How can I resolve this?
 解决方案 
You could forward declare Addition before the base class.
template <typename OperatorType1, typename OperatorType2>
class Addition;
template <typename SubType>
class ArithmeticBase
{
...
};
This allows the compiler to know there is a type Addition that exists before it is defined.
                        
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