runtime - C++ 中的运行时多态性

Question

我有一个接口，我正在尝试一个关于动态多态的示例，如下所示：

#include <iostream>

using namespace std;

class foo{
    public:     
        virtual void set();
        virtual void printValue();
};

class fooInt : public foo{
        private:
            int i;
        public:
            int get(){ 
                return i;
            }   
            void set(int val){ //override the set
                i = val;
            }
            void printValue(){
                cout << i << endl;
            }           
};

int main(){
    foo *dt;         //Create a base class pointer
    dt = new fooInt; //Assign a sub class reference
    dt->set(9);
}

但是，当我编译它时，我没有得到调用 'foo::set(int)' 的匹配函数。我哪里错了？我试图阅读这篇文章，但我仍然无法找出错误。

Answer 1

class foo has no method set(int). It has a method set(), but no method set(int).

If you intend to override an inherited method, the superclass method and your method must have the same signature:

class foo {
  ...
  // If you really want an abstract class, the `= 0`
  //  ensures no instances can be created (makes it "pure virtual")
  virtual void set(int) = 0;
  ...
}

Answer 2

这是因为你的定义

    virtual void set(); 

应该

    virtual void set(int val); 

Answer 3

更正的程序在这里给出

#include <iostream>

using namespace std;

class foo {

public:
    virtual  void set(int val)=0;////////here you have void set() function with no argument but you tried to override void set(int val) which take one argument.
    virtual void printValue()=0;
};

class fooInt : public foo{
private:
    int i;
public:

    fooInt()
    {
        cout<<"constructor called\n";
    }
    int get(){
        return i;
    }
    void set(int val){ //override the set


        i = val;
    }
    void printValue(){
        cout << i << endl;
    }
};

int main(){


    foo *dt;       //Create a base class pointer
    dt=new fooInt;
    dt->set(9);
    dt->printValue();
}

上一个程序的故障是

1.你试图用 set(int val){one argument} 覆盖 set() {no argument}。2.当一个类包含一个纯虚函数时，它必须由它的派生类来实现。3. 不能为包含纯虚函数的类创建对象，但可以创建 ref。谢谢