C++如何將對象添加到地圖和新創建的對象返回內部地圖參考

Question

婁我提供很簡單的東西，我正在掙扎的完整代碼..

• 我需要創建一個地圖字符串和對象...
• 當請求，如果字符串是地圖裏面，我需要一個 參考返回一個對象可在地圖
• 內。當字符串不是地圖裏面，我需要創建對象， 與該字符串並返回（如前）對新的參考 創建對象

請檢查下面的兩條評論我說「錯誤」，看看問題出在哪裏。

我的問題是：

1. 我如何可以插入到地圖，一個對象？ InitObj（）上的行 有什麼問題？
2. 如何創建返回對象的引用，我只是在地圖上創建了 ？截至getHouse（）函數提前

由於年底:)

#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

關於第一個問題看到，你做House不可複製（你的拷貝構造函數和拷貝賦值運算符是私有的）。您採取的方法爲insert需要先製作一個pair，其構造將通過複製House傳入。如果您有權訪問C++ 11編譯器，則仍然可以使用值類型爲地圖是House，只是使用emplace代替：

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

如果您沒有訪問C++編譯器11，你將不得不更改數值類型爲House*或某些等效拷貝構造型。

對於第二個問題，std::map::insert（和emplace）返回pair<iterator, bool>。請利用這一點：

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;