Question

Bellow我提供了一些非常简单的完整代码，我正在努力...

我需要创建一个包含字符串和对象的地图......
当请求时，如果字符串在地图内，我需要返回一个引用地图中的一个对象
当字符串不在地图中时，我需要创建该对象，使用该字符串并返回（如前所述）对新的引用创建对象

请在下面查看我说“错误”的两条评论，以查看问题所在。

我的问题是：

如何将一个对象插入地图？这条线有什么问题在InitObj（）？
如何创建返回对我刚才的对象的引用在地图上创建？如在getHouse（）函数结束时所见

提前致谢：）

#include <map>
#include <string>
#include <memory>

class House
{
public:

    House(const char* name) : _name(name) {};
    ~House() {};

    std::string getHouseName () { return _name; }
private:
    std::string _name;

    House(const House& copy)
    {
    }
    House& operator=(const House& assign)
    {
    }
};

class Obj
{
public:
    Obj()
    {
        InitObj();
    }
    ~Obj() {};

    House& getHouse (const char *houseName)
    {
        std::string name = houseName;
        auto i = _myHouseMap.find(name);

        //this string doesn't exist on map? then create a new house and add to the map and return the reference to it
        if (i == _myHouseMap.end())
        {
            //create a new house
            House h(houseName); 

            //add to the map
            _myHouseMap.insert(std::pair<const std::string, House>(houseName, h));

            //return the reference to the house created
            return h; //<--- ERROR!!!! need to return the reference!
        }
        return (i->second);
    }



private:
    Obj(const Obj& copy);
    Obj& operator=(const Obj& assign);

    typedef std::map<const std::string, House> myHouseMap;

    myHouseMap _myHouseMap;

    //the initialization will add one object to my map
    void InitObj()
    {
        House h("apartment");
        _myHouseMap.insert(std::pair<const std::string, House>("apartment", h)); //<--- ERROR see reference to function template instantiation 'std::pair<_Ty1,_Ty2>::pair<const char(&)[10],House&>
    }
};


int main(void)
{
    Obj aaa;

    House& myHouse1 = aaa.getHouse ("apartment");
    std::cout << "House is " << myHouse1.getHouseName ();

    House& myHouse2 = aaa.getHouse ("newHouse"); //here a new house must be created and added to the map
    std::cout << "House is " << myHouse2.getHouseName ();

    return 0;
}

Answer 1

对于您的第一个问题，您使House不可复制（您的复制构造函数和复制赋值运算符是私有的）。您要对insert采取的方法要求您首先制作pair，其构造将复制您传入的House。如果您有权访问对于C ++ 11编译器，您仍然可以将地图的值类型设为House，而只需使用emplace：

void InitObj()
{
    _myHouseMap.emplace(std::piecewise_construct,
                        std::forward_as_tuple("apartment"),  //key
                        std::forward_as_tuple("apartment")); //value
}

如果您无法访问C ++ 11编译器，则必须将值类型更改为House*或某些等效的可复制构造类型。

对于第二个问题，std::map::insert（和emplace）会返回pair<iterator, bool>。只是利用这个：

if (i == _myHouseMap.end())
{
    House h(houseName); 

    // we insert our new house into the map
    // insert() will return a pair<iterator, bool>. 
    // the bool will be true if the insert succeeded - which we know
    // it will because we know that this key isn't already in the map
    // so we just reassign 'i' to be insert().first, the new iterator
    // pointing to the newly inserted element
    i = _myHouseMap.insert(std::pair<const std::string, House>(houseName, h)).first;
}

// here i either points to the element that was already in the map
// or the new element that we just inserted. either way, 
// we want the same thing
return i->second;

C ++如何将对象添加到地图并返回对地图内新创建对象的引用

1 个答案: