c++ - 如何归还或护理项目 list 基于几个类别

Question

在我的函数“func”中，我想创建一个对象菜单，该菜单返回含柠檬和咖啡的早餐。 'func'返回菜单列表
当我尝试显示仅包含柠檬或仅咖啡的早餐菜单时，它会正确显示。例如:

c.push_back(make_unique<Breakfast>("eggs", 10)); 

但是当我尝试显示还包含柠檬和咖啡的菜单时，它向我显示了主菜单中的错误
这是程序:

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

class Menu {
private:
    int price;
public:
    Menu(int p = 0) : price{ p } {}
    virtual string description() = 0;
    virtual int getPrice() {
        return price;
    }
    virtual ~Menu() {}
};
class WithLemon : public Menu {
private:
    Menu* meniu;

public:
    
    WithLemon(Menu* n) :
        meniu{ n } {}
    string description() override {
        return meniu->description() + " with lemon ";
    }
    int getPrice() override {
        return meniu->getPrice() + 4;
    }
};

class WithCoffee : public Menu {
private:
    Menu* meniu;
public:
    WithCoffee(Menu* n) :
        meniu{ n } {
    }
    string description() override {
        return meniu->description() + " with coffee ";
    }
    int getPrice() override {
        return meniu->getPrice() + 5;
    }
};

class Breakfast : public Menu {
private:
    string name;
public:
    Breakfast(string n, int p) :
        name{ n }, Menu{ p }  {
    }
    string description() override {
        return name;
    }
};
std::vector<std::unique_ptr<Menu>>  func(void)
{
    std::vector <std::unique_ptr<Menu> > c;
    
    
    Breakfast a{ "breakfast eggs", 10 };
    WithCoffee breakfast_with_coffee{ &a };
    Menu* breakfast_with_coffee_and_lemon = new WithLemon{ &breakfast_with_coffee };
    //cout << breakfast_with_coffee_and_lemon->description() << " " << breakfast_with_coffee_and_lemon->getPrice();// print ----> breakfast eggs with coffee  with lemon  19
    
    c.push_back(make_unique<WithLemon>(&breakfast_with_coffee));
    return c;
}
int main() {

    std::vector < std::unique_ptr<Menu> > lista = func();
    for (int i = 0; i < lista.size(); i++) {
        cout << lista[i]->description() << " " << lista[i]->getPrice() << endl; //error read memory access
    }
    return 0;
}

Answer 1

您不能将指针指向自动存储，将其存储在智能指针中并离开该功能。离开该功能后，将释放自动内存，并且智能指针包含一个悬空指针。避免此问题和其他内存泄漏问题的最简单方法是对所有变量使用智能指针:

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

class Menu {
private:
    int price;
public:
    Menu(int p = 0) : price{ p } {}
    virtual std::string description() = 0;
    virtual int getPrice() {
        return price;
    }
    virtual ~Menu() = default;
};

class WithLemon : public Menu {
private:
    std::unique_ptr<Menu> meniu;

public:
    WithLemon(Menu* n) : meniu{ n } {}
    std::string description() override {
        return meniu->description() + " with lemon ";
    }
    int getPrice() override {
        return meniu->getPrice() + 4;
    }
};

class WithCoffee : public Menu {
private:
    std::unique_ptr<Menu> meniu;
public:
    WithCoffee(Menu* n) :
        meniu{ n } {
    }
    std::string description() override {
        return meniu->description() + " with coffee ";
    }
    int getPrice() override {
        return meniu->getPrice() + 5;
    }
};

class Breakfast : public Menu {
private:
    std::string name;
public:
    Breakfast(std::string n, int p) : Menu{ p }, name{ n }  {}
    std::string description() override {
        return name;
    }
};

std::vector<std::unique_ptr<Menu>>  func(void) {
    std::vector <std::unique_ptr<Menu> > c;
    
    auto a = std::make_unique<Breakfast>("breakfast eggs", 10);
    auto breakfast_with_coffee = std::make_unique<WithCoffee>(a.release());
    //Menu* breakfast_with_coffee_and_lemon = new WithLemon{ breakfast_with_coffee };
    //std::cout << breakfast_with_coffee_and_lemon->description() << " " << breakfast_with_coffee_and_lemon->getPrice();// print ----> breakfast eggs with coffee  with lemon  19
    
    c.push_back(std::make_unique<WithLemon>(breakfast_with_coffee.release()));
    return c;
}

int main() {

    std::vector < std::unique_ptr<Menu> > lista = func();
    for (const auto &i : lista) {
        std::cout << i->description() << " " << i->getPrice() << std::endl; //error read memory access
    }
    return 0;
}

避免使用原始的 new和 delete。避免指向自动内存的指针。