如何重写在多个继承中具有相同名称的基类的虚函数? [英] How to override base classes' virtual functions that have identical names in multiple inheritance?

问题描述

假设我有两个基类B1和B2，以及一个从B1和B2派生的类D，如下所示:

Suppose I have two base classes B1 and B2, and a class D that derives from both B1 and B2 as follows:

class B1 {
public:
  // ...
  virtual void foo() final { cout << "Hello, B1\n"; }
};

class B2 {
public:
  // ...
  virtual void foo() { cout << "Good riddance, B2!\n"; }
};

class D :public B1, public B2 {
  // ...
};

在设计类D时，我想从B2覆盖称为foo()的成员函数.但是，B1中的foo()被标记为final，并阻止我覆盖B2中的foo().从B2覆盖foo()的最佳方法是什么?

In designing the class D, I want to override the member function called foo() from B2; however, foo() in B1 is marked final and prevents me from overriding foo() in B2. What is the best approach to override foo() from B2?

推荐答案

我认为您无法按照问题中显示的方式来做.从N3337开始，§10.3/2 [class.virtual]

I don't think what you want to do is possible in the manner you've shown in the question. From N3337, §10.3/2 [class.virtual]

如果在类Base和类Derived中声明了虚拟成员函数vf，这些成员函数直接或间接从Base派生，则成员函数vf具有相同的名称，参数类型-clist(8.3.5)，cv限定词和ref限定词(或没有ref限定词)声明为Base::vf，然后Derived::vf也是虚拟的(无论是否如此声明)，并且它覆盖. ...

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list (8.3.5), cv-qualification, and ref-qualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf. ...

D::foo匹配B1::foo和B2::foo的所有那些条件，因此它将覆盖两者.并且由于B1::foo是final，所以代码格式错误.

D::foo matches all those criteria for B1::foo and B2::foo, hence it overrides both. And since B1::foo is final, the code is ill-formed.

一种解决方法是引入额外的继承级别.定义一个从B2派生并覆盖B2::foo的类，例如D2.然后D可以从B1和D2派生.

One workaround is to introduce an extra level of inheritance. Define a class, say D2, that derives from B2 and overrides B2::foo. Then D can derive from B1 and D2 instead.

class D2 : public B2{
public:
  virtual void foo() override { cout << __PRETTY_FUNCTION__ << '\n'; }
};

class D :public B1, public D2 
{};

D d;
// d.foo();    // error - ambiguous

D2& d2 = d;
d2.foo();   // calls D2::foo
B2& b2 = d;
b2.foo();   // calls D2::foo

B1& b1 = d;
b1.foo();   // calls B1::foo

实时演示

这篇关于如何重写在多个继承中具有相同名称的基类的虚函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！