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c++ - 如何在 C++ 中声明抽象类的向量列表？

问题描述

我有几个类继承自一个主类。为了简单起见，我过度简化了类定义以使其简短直接。

动物.h

所有其他类继承自的主类：

class Animal {
protected:
    string name;
public:
    Animal(string name);
    virtual string toString() { return "I am an animal"; }
};

鸟.h

class Bird: public Animal {
private:
    bool canFly;
public:
    Bird(string name, bool canFly = true) 
     : Animal(name)   // call the super class constructor with its parameter
    {
        this->canFly = canFly;
    }
    string toString() { return "I am a bird"; }
};

检测.h

class Insect: public Animal {
private:
    int numberOfLegs;
public:
    Insect(string name, int numberOfLegs) : Animal(name) {
        this->numberOfLegs = numberOfLegs;
    }
    string toString() { return "I am an insect."; }
};

现在，我需要声明一个vector<Animal>包含每个继承类的多个实例的 a。

主文件

#include <iostream>
#include "animal.h"
#include "bird.h"
#include "insect.h"

// assume that I handled the issue of preventing including a file more than once
// using #ifndef #define and #endif in each header file.

int main() {

    vector<Animal> creatures;

    creatures.push_back(Bird("duck", true));
    creatures.push_back(Bird("penguin", false));
    creatures.push_back(Insect("spider", 8));
    creatures.push_back(Insect("centipede",44));

    // now iterate through the creatures and call their toString()

    for(int i=0; i<creatures.size(); i++) {
        cout << creatures[i].toString() << endl;
    }
}

我期望以下输出：

我是一只鸟

我是一只鸟

我是昆虫

我是昆虫

但我得到了：

我是动物

我是动物

我是动物

我是动物

我知道这与“矢量生物”这一行有关；. It is calling the constructor for 动物. But my intention is to tell the compiler, that this 生物points to an array of动物inherited classes, might be鸟might be昆虫, the point is: they all implement their own unique respective version of toString()`。

我可以做些什么来声明从同一个祖先继承的对象的多态数组？

标签： c++inheritancevectorpolymorphism

解决方案

您不能使用值语义（阅读有关对象切片的信息）。您必须使用指针。

例子：

#include <iostream>
#include <memory>
#include <string>
#include <vector>

class Animal
{
 protected:
  std::string name;

 public:
  Animal(std::string name) : name(name)
  {
  }
  virtual std::string toString()
  {
    return "I am an animal";
  }
  virtual ~Animal()
  {
  }
};

class Bird : public Animal
{
 private:
  bool canFly;

 public:
  Bird(std::string name, bool canFly = true) : Animal(name)  // call the super class constructor with its parameter
  {
    this->canFly = canFly;
  }
  std::string toString()
  {
    return "I am a bird";
  }
};

class Insect : public Animal
{
 private:
  int numberOfLegs;

 public:
  Insect(std::string name, int numberOfLegs) : Animal(name)
  {
    this->numberOfLegs = numberOfLegs;
  }
  std::string toString()
  {
    return "I am an insect.";
  }
};

int main()
{
  std::vector<std::unique_ptr<Animal>> creatures;

  creatures.emplace_back(new Bird("duck", true));
  creatures.emplace_back(new Bird("penguin", false));
  creatures.emplace_back(new Insect("spider", 8));
  creatures.emplace_back(new Insect("centipede", 44));

  // now iterate through the creatures and call their toString()

  for (int i = 0; i < creatures.size(); i++)
  {
    std::cout << creatures[i]->toString() << std::endl;
  }
}

印刷：

I am a bird
I am a bird
I am an insect.
I am an insect.

我还推荐阅读Sean parent Run Time Polymorphism。思路如下：

#include <iostream>
#include <memory>
#include <string>
#include <vector>

class Animal
{
 public:
  struct Interface
  {
    virtual std::string toString() const = 0;
    virtual ~Interface()                 = default;
  };
  std::shared_ptr<const Interface> _p;

 public:
  Animal(Interface* p) : _p(p)
  {
  }
  std::string toString() const
  {
    return _p->toString();
  }
};

class Bird : public Animal::Interface
{
 private:
  std::string _name;
  bool        _canFly;

 public:
  Bird(std::string name, bool canFly = true) : _name(name), _canFly(canFly)
  {
  }
  std::string toString() const override
  {
    return "I am a bird";
  }
};

class Insect : public Animal::Interface
{
 private:
  std::string _name;
  int         _numberOfLegs;

 public:
  Insect(std::string name, int numberOfLegs)
      : _name(name), _numberOfLegs(numberOfLegs)
  {
  }
  std::string toString() const override
  {
    return "I am an insect.";
  }
};

int main()
{
  std::vector<Animal> creatures;

  creatures.emplace_back(new Bird("duck", true));
  creatures.emplace_back(new Bird("penguin", false));
  creatures.emplace_back(new Insect("spider", 8));
  creatures.emplace_back(new Insect("centipede", 44));

  // now iterate through the creatures and call their toString()

  for (int i = 0; i < creatures.size(); i++)
  {
    std::cout << creatures[i].toString() << std::endl;
  }
}

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