派生类的地图 [英] map of derived classes

问题描述

我有一个基类

  class Person {
 public：
 Person（string name， int age）{
 this  - > name = name 
 this  - >年龄=年龄
} 
 virtual void getInfo（）{
 cout<<< Person<<名称<< <年龄; 
} 
 void add（string name，const Person& b）{
 a [name] = b 
} 
 protected：
 string name; 
 int age; 
 map< string，Person>一个; 
}; 
  

包含对象类型Person的地图。我想将各种派生类推送到该地图中，例如

派生类

  class Kid：public Person {
 public：
 Kid（string name，int age）：Person（name，age）{}; 
 virtual void getInfo（）{
 cout<<< Kid<<名称<<< <年龄; 
} 
}; 
  

我想要$ code>添加 Person类的方法bahave如

  Person one（John，25）; 
 one.add（Suzie，15）; 
  

哪个失败。我知道我可以使用指针重写代码，例如

  map< string，Person *> a 
 void add（string name，Person * b）{
 a [name] = b; 
} 
个人（约翰，25岁）; 
 one.add（new Kid（Suzie，15））
  

一种如何实现它而不使用指针？

解决方案

不，不能使用引用或指针，无法获取多态。 / p>

通过认为非指针对象需要存储整个类数据（包括vtable），这个问题很容易理解。

这意味着映射< string，person> 将在<$ c中存储个人 $ c> sizeof（person） slot。

但是一个 sizeof（person）不能包含足够的数据来存储个人的子类的附加信息。这导致对象切片。

I have a base class

class Person{
public:
    Person(string name , int age ){
        this -> name = name;
        this -> age  = age;
    }
    virtual void getInfo(){
        cout << "Person " << name << " " << age;
    }
   void add(string name , const Person & b){
        a[name] = b
   }
protected:
    string name;
    int age;
    map<string , Person > a;
};

That contains map of object type Person. I want to push various derived classes into that map e.g

Derived class

class Kid : public Person{
public:
    Kid(string name, int age):Person(name,age){};
    virtual void getInfo( ){
        cout << "Kid " << name << " " << age;
    }
};

I want add method of Person class to bahave such as

Person one("John",25);
one.add("Suzie",15);

Which fails. I know i can remake the code using pointers e.g

map<string , Person*> a   
void add( string name , Person *b){
      a[name] = b;
}
Person one("John",25);
one.add(new Kid("Suzie",15))

But is there a way how to achieve it without using pointers?

解决方案

No, you can't obtain polymorphism without using references or pointers.

The issue is easily understood by thinking that a non pointer object requires to store the whole class data (including the vtable).

This means that a map<string, person> will store somewhere person instances in a sizeof(person) slot.

But a sizeof(person) can't contain enough data to store additional information of subclasses of person. This leads to object slicing.

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