我可以在基类中重载纯虚方法吗? [英] Can I overload pure virtual method in the base class?

问题描述

在下面的示例中，我有一个抽象类，其中包含纯虚拟方法(aka FUN1)和普通方法(aka FUN2).

In the example below I have a abstract class with pure virtual method (aka FUN1) and a normal method (aka FUN2).

#include <iostream>

class A
{
public:
        virtual void fun(int i) = 0; // FUN1
        void fun() { this->fun(123); } // FUN2
};

class B : public A
{
public:
        virtual void fun(int i) { std::cerr << i << std::endl; }
};

int main(int,char**)
{
        B b;
        b.fun();
}

为什么不能在派生类上调用FUN2? g ++给出错误:

Why can't I call FUN2 on derived class? g++ gives an error:

main.cpp:19:8:错误:没有匹配的函数可以调用"B :: fun()"

main.cpp:19:8: error: no matching function for call to ‘B::fun()’

---

请注意，纯虚拟函数的重载问题是不同的.我不想重写方法.

---

note that Overload of pure virtual function question is different. I don't want to override methods.

推荐答案

这是派生类成员查找工作的方式:在表达式b.fun()中，首先在class B的范围内查找fun，查找找到B::fun(int).因此它停止了，再也找不到A::fun().

This is how derived class member lookup works: in the expression b.fun(), fun is first looked up in the scope of class B, and the lookup finds B::fun(int). So it stops and never finds A::fun().

该标准的相关部分为10.2 [class.member.lookup]/4:

Relevant section of the standard is 10.2 [class.member.lookup]/4:

如果C包含名称为f的声明，则声明集包含在f中声明的每个f声明 C满足在其中进行查找的语言结构的要求. (...)如果结果声明集不为空，则子对象集本身包含C，并且计算完成.

If C contains a declaration of the name f, the declaration set contains every declaration of f declared in C that satisfies the requirements of the language construct in which the lookup occurs. (...) If the resulting declaration set is not empty, the subobject set contains C itself, and calculation is complete.

要直接访问基类函数，可以在派生类中使用using声明，即using A::fun;.

To make the base class function directly accessible you can use a using declaration in the derived class, i.e. using A::fun;.

对于在基类中实现的方法，有时可以选择调用b.A::fun().

For methods that are implemented in the base class an alternative is sometimes to qualify to call, i.e. b.A::fun().

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