Question

我想知道如何实现以下设计。我没有努力遵循标准惯例，以便将这个虚拟代码保持在最低限度（例如，包括警卫）。为了论证，我使用的是C ++ 03，只使用标准库组件。

TL; DR ：我的Restaurant和Recipe类之间存在循环依赖关系。我在std::set<>类中有Recipe对象的Restaurant个#include，反之亦然。我不能value_type一个在另一个，但由于前向声明，编译器不允许我为集合声明自定义比较器，因为此时/* * restaurant.h */ #include <string> #include <set> /* * I cannot do this as recipe.h would need to include * restaurant.h, which restricts * me when I want to write a custom comparator. */ // #include "recipe.h" class Recipe; // I know this will not work as Recipe has not yet been declared. struct RecipeComparator { bool operator() (const Recipe* lhs, const Recipe* rhs) { return lhs->price() < rhs->price(); } }; class Restaurant { public: Restaurant(const std::string& name, float averagePrice); float averagePrice() { return m_averagePrice; } private: std::string m_name; float m_averagePrice; /* * I want to have the recipies sorted by their prices. * I cannot define RecipeComparator here as Restaurant * is an incomplete type till now. */ std::set< Recipe*, RecipeComparator > m_recipiesSorted; /* * This works, but does not do what I want. */ std::set< Recipe* > m_recipies; };仍然是不完整的类型。有没有办法实现这个目标？

我在代码中添加了注释，可以更详细地解释我的问题。

/*
 * recipe.h
 */
#include <string>
#include <set>
/*
 * I cannot do this as restaurant.h would need to include
 * recipe.h, so I need to forward declare, which restricts
 * me when I want to write a custom comparator.
 */
// #include "restaurant.h"

class Restaurant;

// I know this will not work as Restaurant has not yet been declared.
struct RestaurantComparator {
    bool operator() (const Restaurant* lhs, const Restaurant* rhs) { 
        return lhs->averagePrice() < rhs->averagePrice();
    }
};

class Recipe {
    public:
        Recipe(const std::string& name, float price);
        float price() { return m_price; }

    private:
        std::string m_name;
        float m_price;
        /*
         * This is what I want as I want to have the restaurants sorted
         * by their average prices.
         * I cannot define RestaurantComparator here as Restaurant
         * is an incomplete type till now. 
         */
        std::set< Restaurant*, RestaurantComparator > m_restaurantsSorted;

        /*
         * This works, but does not do what I want.
         */
        std::set< Restaurant* > m_restaurants;

};

{{1}}

Answer 1

一个想法是将Comparator定义移动到.cpp源，同时保持标头中的声明。这样，restaurant.h和recipe.h仍然只会通过名称互相引用，因此前向声明仍然足够。

// restaurant_recipe_comparators.h
// probably too long, give it a better name
struct RestaurantComparator
{
    bool operator() (const Restaurant *, const Restaurant *);
};

struct RecipeComparator
{
    bool operator() (const Recipe *, const Recipe *);
};

// restaurant_recipe_comparators.cpp
bool RestaurantComparator::operator() (const Restaurant* lhs, const Restaurant* rhs)
{ 
    return lhs->averagePrice() < rhs->averagePrice();
}


bool RecipeComparator::operator() (const Recipe* lhs, const Recipe* rhs)
{
    return lhs->price() < rhs->price();
}

编译C ++多对多的类关系

1 个答案: