c++ - 如何使用 friend 类但引入较少的依赖

Question

项目 A，PA.h:

#include "Common.h"

class PA
{
   void func()
   {
     Common::getInstance()->cm();
     Common::getInstance()->onlyCallByPA();
   }
}

项目通用，Common.h:

class Common
{
    SINGLETON
public:
  void cm(){}
 private:
    //I do not want PB to call onlyCallByPA
    //so I want to add class PA to friend class
    //so I need to include PA.h
    //but if I include PA.h, PB.cpp  include PA.h 
    //this will make PA.h expose to PB
    //I do not want PB to include PA.h
  void onlyCallByPA(){}
}

项目 B，PB.cpp:

#include "Common.h"

class PB
{
    //I need to call cm() but PB do not be allowed to call onlyCallByPA
    //and also do not be allowed to include PA.h
}

所以我想让PA成为Common的友元类，但是这会引入对PB的依赖。
有更好的解决方案吗？或者，我可以使用其他设计来实现我想要的吗？

Answer 1

使用前向声明。这将允许您在不包含 header 或依赖 PA 的完整类声明的情况下声明友元。

通用.h

class PA; // forward declaration.

class Common
{
    friend PA;
};