虚拟继承，一类足够？ [英] Virtual Inheritance, one class enough?

问题描述

我理解虚拟继承的概念，但我无法在任何地方找到答案。说你有继承类B和C的类D. B和C都继承类A.所以你可以使B和C虚拟地继承A避免A的两个实例。但是你必须在B和C指定虚拟继承

感谢

div class =h2_lin>解决方案

它们都必须 virtual 。从C ++ 11 10.1 [class.mi] / 7：

类可以同时拥有给定类型的虚拟和非虚拟基类。

  class B {/ * ... * /}; 
 class X：virtual public B {/ * ... * /}; 
 class Y：virtual public B {/ * ... * /}; 
 class Z：public B {/ * ... * /}; 
 class AA：public X，public Y，public Z {/ * ... * /}; 
  

对于 AA 在 AA 的类晶格中的基类 B 的虚拟出现对应于单个 / code>子类型 AA 的对象内的子对象，以及每个其他出现的（非虚拟）基类 B AA 类类中的c $ c>对应一对一与一个不同的 B 子对象 AA 类型。给定上面定义的类 AA ，类 AA 有两个子类 B ： Z 的 B 和虚拟 B 分享 X 和 Y ，如下所示。

I understand the concept of virtual inheritance, but I couldn't find the answer to this anywhere. Say you have class D which inherits class B and C. Both B and C inherit class A. So you could make B and C virtually inherit A to avoid two instances of A. But do you have to specify virtual inheritance at both B and C or does it already create only one instance of A if one of the two virtually inherits A and the other doesn't?

Thanks

解决方案

They must all be virtual. From C++11 10.1 [class.mi]/7:

A class can have both virtual and non-virtual base classes of a given type.

class B { /* ... */ };
class X : virtual public B { /* ... */ };
class Y : virtual public B { /* ... */ };
class Z : public B { /* ... */ };
class AA : public X, public Y, public Z { /* ... */ };

For an object of class AA, all virtual occurrences of base class B in the class lattice of AA correspond to a single B subobject within the object of type AA, and every other occurrence of a (non-virtual) base class B in the class lattice of AA corresponds one-to-one with a distinct B subobject within the object of type AA. Given the class AA defined above, class AA has two subobjects of class B: Z’s B and the virtual B shared by X and Y, as shown below.

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