从基类实例调用派生类方法

Question

我正在尝试使用多态性，从基类实例调用派生的“add”方法。

但它仍然想运行基类 RealThing "add" 方法。

我期待派生类 RealThingNextVersion "add" 方法。

#include <iostream>
#include <string>
using namespace std;

class VirtualObject
{
public:
    virtual void load() {
        cout << "Nothing to load." << endl;
    }

    void import(string file) {
        //here some importing stuff
    }
};

class RealThing :public VirtualObject
{
private:
    string file;

protected:
    int id;

public:
    RealThing(string fileName = "data.txt") { file = fileName; }

    void load() {
        this->import(this->file);
        cout << "Thing data loaded." << endl;
    }

    virtual void add(int id) {
        cout << "Added V1: " << id << endl;
    };
};

class RealThingNextVersion : public RealThing
{
public:
    string desc;

public:
    virtual void add(int id, string desc) {
        cout << "Added V2: " << id << " with description " << desc <<  endl;
    };
};


int main() {
    RealThing rt;
    RealThingNextVersion rtv;

    RealThing* r;
    r = &rt;
    r->load(); // OK., returns: Thing data loaded.
    r->add(11); // OK., returns: Added V1: ...

    r = &rtv;
    r->add(22, "info"); // Here "Error: too many arguments in function call" 
    // it still want's to run RealThing "add",
    // and I was expecting RealThingNextVersion "add"


    system("pause");
}

Answer 1

RealThingNextVersion 中的add 与RealThing 中的调用签名（不同的参数）不同，因此它是不同的函数，而不是add 的覆盖。如果在函数声明中添加 override 关键字

virtual void add(int id, string desc) override;

使用最近的编译器，编译器会告诉你你没有覆盖任何东西。