c++ - 多重虚继承

Question

我在工作时在我们的代码库中发现了以下代码，我觉得它看起来很奇怪。类 RetainCounting 用于实现引用计数。

1.IProvider真的应该派生自RetainCounting吗？如果它只是一个抽象基类，然后让实现 IProvider 的类(即 ProviderImpl)从“RetainCounting”派生(即，如果它需要保留/释放功能)不是更好吗？

2.如果我们认为IProvider应该派生自RetainCounting，为什么ProviderImpl还需要派生自“RetainCounting”，而IProvider已经有这个继承？

class RetainCounting
{
public:
   virtual ~RetainCounting();
   void retain();
   void release();

private:
   // more code....
};

class IProvider : public virtual RetainCounting
{
public:
   virtual void doSomething() = 0;
};

class ProviderImpl : virtual private IProvider,
                     virtual public RetainCounting
{
public:
   void doSomething() override;
};

Answer 1

2.If we believe that IProvider should derive from RetainCounting, why do ProviderImpl needs to derive from "RetainCounting" as well, when the IProvider already has this inheritance?

如果它必须公开继承 RetainCounting，那么它必须继承它，因为 IProvider - 并且传递它自己的 RetainCounting 基础 - 是私有(private)继承的.但是您可以非虚拟地继承 IProvider，除非某些东西继承了多个 IProvider 派生词。

如果不需要可见性差异，那么可以通过去掉虚继承，只继承IProvider来简化设计。

class IProvider : public RetainCounting

class ProviderImpl : public IProvider // or private, depending on what you need

1.Should the IProvider really derive from RetainCounting? Isn´t it better if it was just an abstract base class and then let the class that implements IProvider i.e. ProviderImpl derive from the "RetainCounting" instead (i.e. if it needs retain/release features)?

如果 IProvider 的所有派生类都不需要引用计数，那么 IProvider 不应该继承 RetainCounting 而应该只而是在导数中完成。

接口(interface)通常是指一个只有纯虚函数，没有成员变量的类。如果 IProvider 继承了一些不是接口(interface)的东西，那么它本身就不是接口(interface)。

如果 IProvider 的所有派生词都必须进行引用计数，但您希望 IProvider 成为一个接口(interface)，例如通常以 I 为前缀名称，那么您可以做的是定义一个用于引用计数的接口(interface)。我们称它为 IRetainCounting。由于实现可能由许多衍生产品共享，因此您可以创建一个实现它的基类。这导致您需要虚拟继承。

class IRetainCounting
{
public:
    virtual ~IRetainCounting(){}
    virtual void retain() = 0;
    virtual void release() = 0;
};

class RetainCounting: public virtual IRetainCounting
{
public:
    void retain() override;
    void release() override;

private:
   // more code....
};

class IProvider : public virtual IRetainCounting
{
public:
    virtual void doSomething() = 0;
};

// option 1
class ProviderImpl : private IProvider,
                     public RetainCounting
{
public:
    void doSomething() override;
};

// option 2, you can keep BaseRetainCounting private if you want to
// I recommend this if there can be more than 1 implementation of IRetainCounting
// otherwise 1 is simpler
class ProviderImpl : private IProvider,
                     private RetainCounting,
                     public virtual IRetainCounting

优点:纯接口(interface)

缺点:代码更多(但数量不变。样板文件的数量不会随着衍生品的数量增加。)和虚拟继承的需要(无论如何你都需要虚拟继承来实现可见性差异)