保护/私人继承铸造 [英] Protected/Private Inheritance Casting

问题描述

当我去继承和上/下投射时，我偶然遇到了这个问题。为什么不允许这样做（代码被注释为显示不允许的部分）？

I came across this issue accidentally when I was going over inheritance and up/down casting. Why is this not allowed (code is commented to show sections that are not allowed)? Now I can guess as to why it is not allowed but a factual answer would be great.

至于允许的代码，我可以猜测为什么不允许这样做知道这是因为（Base *）是一个C风格的转换，它本质上是C ++中的一个 reinterpret_cast ，这反过来意味着在这种情况下它会导致未定义的行为。如果我错了，请更正我。

As for the code that is allowed, I know it is because (Base*) is a C-style cast which is essentially a reinterpret_cast in C++ which in turn means that in this case it will result in undefined behavior. Please correct me if I am wrong.

class Base
{
};

class Derived : public Base
{
};

class DerivedProt : protected Base
{
};

class DerivedPriv : private Base
{
};

int main()
{
  Base* a = new Derived();
  Base* b = new DerivedProt();  // Not allowed
  Base* c = new DerivedPriv();  // Not allowed

  Base* d = (Base*) new DerivedProt(); // Allowed but undefined behavior...?
  Base* e = (Base*) new DerivedPriv(); // Allowed but undefined behavior...?
}

推荐答案

听起来你是正确的。

需要记住的一点是，传统的OO原则，如LSP只描述公共继承。非公共继承属于继承和组合之间，基本子对象是非公共的，像组合，但你也可以利用依赖于继承的功能，如虚函数。

One thing to remember is that traditional OO principles such as the LSP only describe public inheritance. Non-public inheritance falls in-between inheritance and composition, the base subobject is non-public like composition, but you can also take advantage of features which rely on inheritance, such as virtual functions.

但是，就像一个组成的子对象，只有类（或其后代，在受保护的继承的情况下），可以获得子对象的地址。

Just like a composed subobject, however, only the class (or its descendants, in case of protected inheritance), can get the address of the subobject.

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