有沒有辦法專門

Question

我有兩個專門的對象繼承對象的一般方法：

class Food {}; 
class Fruit : public Food {}; 
class Vegetable : public Food {}; 

然後我將繼承父類：

class Parent 
{ 
    virtual void say(Food* obj) { std::cout << "The object is food" << std::endl; } 
}; 

而且繼承來自父母的課程。

class Child : public Parent 
{ 
    virtual void say(Fruit* obj) { std::cout << "The object is a fruit" << std::endl; } 
    virtual void say(Vegetable* obj) { std::cout << "The object is a vegetable" << std::endl; } 
}; 

我做的：

std::vector<Food*> basket; 
Fruit fruit = Fruit(); 
Vegetable vegetable = Vegetable(); 
basket.push_back(&fruit); 
basket.push_back(&vegetable); 

Child child = Child(); 

for (Food* food : basket) 
{ 
    child.say(food); 
} 

我想它打印「的對象是一種水果」，然後「的對象是蔬菜」，但它不工作： 我得到錯誤消息：參數1從'Food *'到'Fruit *'沒有已知的轉換。

有沒有辦法做到這一點，如果可能的話，不使用typeid，因爲我聽到它會導致開銷。這裏是一個在線編輯器代碼：cpp.sh/27ekc

Answer 1

我認爲合適的解決方案，這是以下幾點：

class Food 
{ 
public: 
    virtual ~Food() = default; 
    virtual void say() const; 
}; 
class Fruit : public Food 
{ 
public: 
    void say() const override { std::cout << "The object is a fruit" << std::endl; } 
}; 
class Vegetable : public Food 
{ 
public: 
    void say() const override { std::cout << "The object is a vegetable" << std::endl; } 
}; 

class Parent 
{ 
public: 
    virtual ~Parent() = default; 
    virtual void say(const Food& obj) const { obj.say(); } 
}; 

class Child : public Parent {}; 

int main() 
{ 
    std::vector<Food*> basket; 
    Fruit fruit = Fruit(); 
    Vegetable vegetable = Vegetable(); 
    basket.push_back(&fruit); 
    basket.push_back(&vegetable); 

    Child child = Child(); 

    for (const Food* food : basket) 
    { 
     child.say(*food); 
    } 
} 

編輯： 按您的評論，它是根據你所說的健康是什麼。 我把它解釋爲這些方針的東西：

class Food 
{ 
public: 
    virtual ~Food() = default; 
    virtual void say() const; 
    virtual int health() const; 
}; 
class Fruit : public Food 
{ 
public: 
    void say() const override { std::cout << "The object is a fruit" << std::endl; } 
    int health() const override { return 5; } 
}; 
class Vegetable : public Food 
{ 
public: 
    void say() const override { std::cout << "The object is a vegetable" << std::endl; } 
    int health() const override { return 10; } 
}; 

class Parent 
{ 
public: 
    virtual ~Parent() = default; 
    virtual void say(const Food& obj) const { obj.say(); } 
}; 

class Child : public Parent 
{ 
    int health; 

public: 
    void eat(const Food& obj) { health += obj.health(); } 
}; 

int main() 
{ 
    std::vector<Food*> basket; 
    Fruit fruit = Fruit(); 
    Vegetable vegetable = Vegetable(); 
    basket.push_back(&fruit); 
    basket.push_back(&vegetable); 

    Child child = Child(); 

    for (const Food* food : basket) 
    { 
     child.say(*food); 
     child.eat(*food); 
    } 
} 

有很多不同的方式來實現這一目標。