如何检测和断言特定类的虚拟继承？

发布于 2024-12-21 19:01:44 字数 114 浏览 2 评论 0原文

我有一个实现引用计数的 C++ 类，并且我希望该类的所有用户仅从该类虚拟继承，以便没有对象最终拥有多个引用计数器。

我想要某种方法来在编译时或至少在运行时断言此要求。

有办法实现吗？

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乖乖哒 2024-12-28 19:01:44

像这样的东西吗？

struct RefCounter {
    template <typename T>
    RefCounter(T *) {
        BOOST_STATIC_ASSERT(boost::is_virtual_base_of<RefCounter, T>);
    }
};

struct GoodClass : virtual RefCounter {
    GoodClass() : RefCounter(this) {}
};

struct BadClass : RefCounter {
    BadClass() : RefCounter(this) {}
};

不过，需要将 this 传递给构造函数来捕获派生类型，这很遗憾。当然，故意迟钝的用户可以通过传递 this 以外的其他内容来破坏它。

Something like this?

struct RefCounter {
    template <typename T>
    RefCounter(T *) {
        BOOST_STATIC_ASSERT(boost::is_virtual_base_of<RefCounter, T>);
    }
};

struct GoodClass : virtual RefCounter {
    GoodClass() : RefCounter(this) {}
};

struct BadClass : RefCounter {
    BadClass() : RefCounter(this) {}
};

It's a shame about needing to pass this to the constructor, though, to capture the derived type. And of course a wilfully obtuse user could subvert it by passing something other than this.

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缘字诀 2024-12-28 19:01:44

我认为包装类将是最简单的选择。而不是直接继承自 RefCounter 创建一个中间类。

struct RefCounterVirtPrivate_
{
    int count;

    RefCounterVirt()
        : count( 0 )
    { }
};

struct RefCounter : public virtual RefCounterVirtPrivate_
{
};

struct A : public RefCounter { };
struct B : public RefCounter { };
struct C : public A, public B { };

那么一切都可以从 RefCounter 继承，而不需要关心虚拟继承。您甚至不必更改任何现有代码——RefCounter 本身的虚拟继承应该是无害的。

这当然并不能保证人们不会直接继承RefCounterVirtPrivate_，但这就是为什么我给它一个明显的名称。意外执行此操作比忘记 virtual 关键字更难。

I think wrapping the class would be the simplest option. Rather than directly inheriting from RefCounter create an intermediary class.

struct RefCounterVirtPrivate_
{
    int count;

    RefCounterVirt()
        : count( 0 )
    { }
};

struct RefCounter : public virtual RefCounterVirtPrivate_
{
};

struct A : public RefCounter { };
struct B : public RefCounter { };
struct C : public A, public B { };

Then everything can inherit from RefCounter without any need to care about virtual inheritence. You don't even have to change any existing code -- virtual inheritence of RefCounter itself should be harmless.

This of course doesn't guarantee people don't inherit from RefCounterVirtPrivate_ directly, but that is why I've given it an obvious name. It's harder to accidentally do this than forget a virtual keyword.

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