使用C ++，如何正确地两次从同一个基类继承? [英] Using C++, how do I correctly inherit from the same base class twice?

问题描述

这是我们理想的继承层次结构:

This is our ideal inheritance hierarchy:

class Foobar;

class FoobarClient : Foobar;

class FoobarServer : Foobar;

class WindowsFoobar : Foobar;

class UnixFoobar : Foobar;

class WindowsFoobarClient : WindowsFoobar, FoobarClient;

class WindowsFoobarServer : WindowsFoobar, FoobarServer;

class UnixFoobarClient : UnixFoobar, FoobarClient;

class UnixFoobarServer : UnixFoobar, FoobarServer;

这是因为我们的继承层次结构会尝试从Foobar继承两次，因此，编译器会抱怨Foobar的任何成员上的引用不明确.

This is because the our inheritance hierarchy would try to inherit from Foobar twice, and as such, the compiler would complain of ambiguous references on any members of Foobar.

允许我解释为什么我想要一个如此复杂的模型.这是因为我们希望从WindowsFoobar，UnixFoobar，FoobarClient和FoobarServer访问相同的变量.这不会有问题，只有我想对以上任意组合使用多重继承，以便我可以在任何平台上使用服务器/客户端功能，也可以在客户端或服务器上使用平台功能.

Allow me to explain why I want such a complex model. This is because we want to have the same variable accessible from WindowsFoobar, UnixFoobar, FoobarClient, and FoobarServer. This wouldn't be a problem, only I'd like to use multiple inheritance with any combination of the above, so that I can use a server/client function on any platform, and also use a platform function on either client or server.

我忍不住觉得这是一个多重继承的普遍问题……我是从完全错误的角度来解决这个问题吗?

I can't help but feel this is a somewhat common issue with multiple inheritance... Am I approaching this problem from completely the wrong angle?

此外，请考虑我们可以使用#ifdef来解决此问题，但是，这样会产生非常丑陋的代码，如下所示:

Also, consider that we could use #ifdef to get around this, however, this will tend to yield very ugly code like such:

CFoobar::CFoobar()
#if SYSAPI_WIN32
: m_someData(1234)
#endif
{
}

...好吧！

对于那些想深入了解此问题背景的人，我真的建议略读适当的邮件列表线程.在第3个帖子周围，事情开始变得有趣起来.另外，您还可以进行相关代码提交在此处查看有问题的真实代码.

For those who want to read more into the background of this issue, I really suggest skimming over the appropriate mailing list thread. Thing start to get interesting around the 3rd post. Also there is a related code commit with which you can see the real life code in question here.

推荐答案

它可以正常工作，尽管您将获得基类Foobar的两个副本.要获得一个副本，您需要使用虚拟继承.在此处上阅读.

It would work, although you'd get two copies of the base Foobar class. To get a single copy, you'd need to use virtual inheritance. Read on multiple inheritance here.

class Foobar;

class FoobarClient : virtual public Foobar;

class FoobarServer : virtual public Foobar;

class WindowsFoobar : virtual public Foobar;

class UnixFoobar : virtual public Foobar;

但是，与多重继承相关的问题很多.如果您真的想展示模型，为什么不让FoobarClient和FoobarServer在构造时引用Foobar，然后使用Foobar& FoobarClient/Server::getFoobar?

However, there are many problems associated with multiple inheritance. If you really want to have the model presented, why not make FoobarClient and FoobarServer take a reference to Foobar at construction time, and then have Foobar& FoobarClient/Server::getFoobar ?

组合通常是摆脱多重继承的一种方法.现在举个例子:

Composition is often a way out of multiple inheritance. Take a example now:

class WindowsFoobarClient : public WindowsFoobar 
{
    FoobarClient client;
public:
    WindowsFoobarClient() : client( this ) {}
    FoobarClient& getClient() { return client }
}

但是在使用此功能时，必须要小心在构造函数中.

However care must be taken in using this in the constructor.

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