c++ - C++ 类的静态数据成员的多态性

Question

能否在 C++ 类(又名“类变量”)的静态数据成员上使用多态性？

更新:正如 b4hand 所述，必须在基类中声明多态方法。但在这种情况下这是不可能的，因为事先不知道用户将在 containerDerived 中创建什么方法。

我编辑了示例以包含引用；谢谢你提醒我。

这是我尝试将静态基类型 (numBase) 初始化为派生类型 (numDer) 的尝试:

#include <iostream>

class numBase                   //numBase is in a library user can not edit
{
};

class numDer : public numBase   //user defined class is a kind of numBase
{
    private:
        int num;
    public:
        void printNum() { std::cout << " numDer=" << num; }
        void inc() { num++; }
};

class containerBase             //containerBase is in a library user can not edit
{
    protected:
        //static numDer& count; //this compiles, but count can not be initialized to other types
        static numBase& count;  //this causes error six lines down from here
};

class containerDerived : public containerBase //user defined class is a kind of containerBase
{
    public:
        void inc() { count.inc(); } //error: 'count' was not declared in this scope
        void printCount() { std::cout << " containerDerived"; count.printNum(); }
};

/************************ user program **********************/
//initilialize static variable
numDer number;              //number could be any user defined type derived from numBase
numDer& containerBase::count = number; //initialize count to a kind of numBase

int main()
{
    containerDerived container1;
    containerDerived container2;

    container1.printCount();
    container1.inc();
    container2.inc();
    container1.printCount();
}

谢谢。

Answer 1

static numBase count;   

您已将 count 声明为 numBase 的对象。通过该对象，您试图调用 numBase 派生类的成员，即 numDer.inc()。此调用肯定不会成功，因为运行时多态性仅适用于使用指针/引用的情况。