c++ - 从抽象(纯虚拟)类私有(private)继承是否有意义？

Question

假设这个结构

struct InterfaceForFoo
{
    virtual void GetItDone() = 0;
};


class APoliticallyCorrectImplementationOfFooRelatedThings : private InterfaceForFoo
{

  public:
    void GetItDone() { /*do the thing already*/ };   
};

现在，我想知道以这种方式从接口(interface)私有(private)继承是否有任何有用的场景。

Answer 1

哈，这里的每个人都说“不”。我说“是的，它确实有意义。”

class VirtualBase {
public:
    virtual void vmethod() = 0;
    // If "global" is an instance of Concrete, then you can still access
    // VirtualBase's public members, even though they're private members for Concrete
    static VirtualBase *global;
};

// This can also access all of VirtualBase's public members,
// even if an instance of Concrete is passed in,
void someComplicatedFunction(VirtualBase &obj, ...);

class Concrete : private VirtualBase {
private:
    virtual void vmethod();
public:
    void cmethod() {
        // This assignment can only be done by Concrete or friends of Concrete
        VirtualBase::global = this;
        // This can also only be done by Concrete and friends
        someComplicatedFunction(*this);
    }
};

使继承private并不意味着你不能从类外访问VirtualBase的成员，它只意味着你不能访问那些成员通过对 Concrete 的引用。但是，Concrete 及其 friend 可以将 Concrete 的实例转换为 VirtualBase，然后任何人都可以访问公共(public)成员。简单地说，

Concrete *obj = new Concrete;
obj->vmethod(); // error, vmethod is private

VirtualBase *obj = VirtualBase::global;
obj->vmethod(); // OK, even if "obj" is really an instance of Concrete