c++ - 变量 : Private vs Protected 的访问修饰符

Question

我有一个案例，我找不到合适的解决方案。我需要有模板基类和一个对该基类做一些工作的管理器类。这是代码

class Base;

class Manager
{
public:
    void DoSomethingTo(Base * bclass)
    {
        if(bclass->PrivateVar == 0)
            // Some code
    }
}

class Base
{
protected: 
    int PrivateVar;
friend class Manager;
};

template<class T>
class TempBase : public Base
{
private:
    Manager * man;
public:
    void DoWork()
    {
        PrivateVar = 0;
        man->DoSomethingTo(this);
    }
}

我的问题是 PrivateVar 变量实际上必须是 TempBase 私有(private)的，并且不能从派生自 TempBase 的类访问，但是因为我不能将它的指针传递给 Manager::DoSomethingTo()，所以我必须从基类派生它，基类必须拥有 PrivateVar。由于我在 TempBase 中使用 PrivateVar，它不能对 Base 私有(private)，这使得它可以从 TempBase 的子类访问。

我如何编写 TempBase 类，使 PrivateVar 成为私有(private)成员，而它可以从 Manager::DoSomethingTo() 访问？

谢谢

Answer 1

首先，让我们开始火焰大战:

There is never a reason for an attribute of a class to be protected. It shall either be private (in most cases) or perhaps in some corner cases public. Note that because every derived classes would see a protected attribute, protected only brings a false sense of security: one cannot control any invariant on a protected attribute, so it's essentially a public one, with a comment Don't Touch on top of it.

既然已经说完了，我建议简单地将其更改为:

class Base {};
template <typename T> class TBase;

class DoKey { template <typename T> friend class TBase; DoKey(); };

class Manager
{
public:
  void DoSomethingTo(Base& base, DoKey const& key);
};

template <typename T>
class TBase: public Base
{
public:
  void Do()
  {
    Manager manager;
    if (PrivateVar == 0) { manager.DoSomething(*this, DoKey()); }
  }

private:
  int PrivateVar;
};

通过将测试的责任委托(delegate)给变量的所有者，我们轻轻地绕开了所有的担忧。当然，如果我们需要多个类以类似的方式运行，那么我们可以在 Base 中引入一个纯虚拟的 void ShouldIDoSomething() const 并在 中检查它经理的方法。