是否可以为复制基的虚函数提供不同的定义？

Question

我知道声明， "A virtual function of a replicated base class can be overridden by a (single) function in a derived class." 我很好奇是否可以提供多个定义，以防万一——如何正确定义和调用？

为了更清楚一点，我的 classes 和 class 层次结构就像

     A  A
     |  |
     B  C
     \ /
      D

//i.e., I am not deriving B and C from a virtual base A.

class A {
    virtual void f(){
         cout<<"belongs to A"<<endl;
    }
}
class B: public A {
    void f(){
         cout<<"belongs to B"<<endl;
    }
}
class C: public A {
    void f(){
         cout<<"belongs to C"<<endl;
    }
}
class D: public B, public C {
    void f(){  //This overrides definitions B::f() and C::f()
         cout<<"belongs to D"<<endl;
    }
}
B* b;
C* c;
D d;
b = &d;
c = &d;
b->f();//This would output "belongs to D"
c->f();//This also would output "belongs to D"

//Now I want to redefine f() in D like this
class D: public B, public C {
    void B::f() {
         cout<<"belongs to D::B::f()"<<endl;
    }
    void C::f() {
         cout<<"belongs to D::C::f()"<<endl;
    }
}
//So that 
B* b;
C* c;
D d;
b = &d;
c = &d;
b->f();//This would output "belongs to D::B::f()"
c->f();//This also would output "belongs to D::C::f()"

Answer 1

在某些时候，您需要能够告诉编译器您要使用哪个实现。

这样做的一种方法：

  A
 / \
B   C
 \ /
  D

struct A
{
  virtual void f();
};

struct B : public A
{
  void f() override;
};

struct C : public A
{
  void f() override;
};

struct D : public B,C
{
};

int test(int num) {

  D d;
  d.f();    // undefined
  d.A::f(); // Impossible because conversion from D to A is ambiguous
  d.B::f(); // calls B implementation
  d.C::f(); // calls C implementation
}

Answer 2

嗯，下面是一个虚函数的基本例子

class Person
{
    int age;
    public:
        Person(int=0);
        virtual void describe() = 0;
        int getAge();
};

Person :: Person(int age)
{
    this->age = age;
}

int Person :: getAge()
{
    return this->age;
}

class Student : public Person
{
    public:
        Student(int=0);
        void describe();
};

Student :: Student(int age) : Person(age) {}
void Student :: describe() 
{
    std::cout << "Hi, I am a student, ";
    std::cout << this->getAge() << " years old!";
}

class Employee : public Student
{
    public:
        Employee(int=0);
        void describe();
};

Employee :: Employee(int age) : Person(age) {}
void Employee :: describe()
{
    std::cout << "Hi, I am an employee, ";
    std::cout << this->getAge() << " years old!";
}

Answer 3

简短的回答是否。 – 好奇的家伙

@curiousguy 在评论中提供的答案。

是否可以为复制基的虚函数提供不同的定义？

Is it possible to provide different definitions for a virtual function of a replicated base?

c++

overriding

virtual-functions

multiple-inheritance