在不破坏现有基类的情况下向具有许多派生类的抽象基类添加新方法

Question

我有一个抽象基类。有很多抽象类派生自这个抽象基类

    #if ABI_VERSION_ATLEAST1
    class Base
    {
    public:
      virtual void food() =0
    };
  #endif

例如

    #if ABI_VERSION_ATLEAST1
    class Derived1: public Base
    {
    };
   #endif

假设我想在不破坏二进制兼容性的情况下为其添加新方法。 唯一的办法就是扩展这个类

#if ABI_VERSION_ATLEAST2
    class Base_Ext : public Base
    {
    public:
      virtual void food1()=0;
    };
    #endif

问题是已经存在的派生类实现将无法访问这个 food1()。 怎么解决这个问题 抽象派生类怎么能看到这个新方法

我想到的一个解决方案是：-

我需要扩展 Derived1.......

#if ABI_VERSION_ATLEAST2

class Derived2 : public Derived1, public Base_Ex
{ 
} ;

#endif 

再次在这里解决钻石问题，我将不得不改变

class Derived1: public Base to

 class Derived1: public virtual Base {} 

我不想这样做..因为它会再次破坏现有派生类的二进制兼容性。所以坚持下去

Answer 1

您可以创建第二个基类，我们将其命名为OtherBase，您可以在其中添加新方法。 你可以创建新的派生类，我们命名为Derived2，它继承了OtherBase和Derived1的方法：

#include <iostream>

class Base
{
public:
  virtual void food() = 0;
  virtual ~Base() {}; // virtual destructor is necessary to avoid memory leak
};

class Derived1 : public Base
{
public:
    virtual void food() override
    {
        std::cout << "Derived1::food" << std::endl;
    }
    virtual ~Derived1() override {}
};

class OtherBase
{
public:
    virtual void food1() = 0;
    virtual ~OtherBase() {}; // virtual destructor is necessary to avoid memory leak
};

class Derived2 : public OtherBase, public Derived1
{
public:
    virtual void food1() override
    {
        std::cout << "Derived2::food1" << std::endl;
    }
    virtual ~Derived2() override {}
};

int main()
{
    std::cout << "On the stack:" << std::endl;
    Derived2 derived2;
    derived2.food();
    derived2.food1();
    
    std::cout << "On the heap:" << std::endl;
    OtherBase * otherBase_p1 = new Derived2();
    if (otherBase_p1) // check if memory was is allocated
    {
        otherBase_p1->food1();
        
        Base * base_p = dynamic_cast<Base *>(otherBase_p1);
        if (base_p) // if dynamic_cast was unsuccessful than base_p is nullptr
        {
            base_p->food();
        }
        
        delete otherBase_p1;
    }
}