c ++通过引用将映射传递给函数 [英] c++ pass a map by reference into function

问题描述

如何将地图通过引用传入函数？ Visual Studio 2010给我一个未解决的外部错误。目前，我有以下简化代码：

How can I pass a map by reference into a function? Visual Studio 2010 is giving me an unresolved externals error. Currently, I have the following simplified code:

void function1(){
    map<int, int> * my_map = new map<int, int>(); 
    function2(*my_map); 
}

void function2(map<int, int> &temp_map){
    //do stuff with the map
}

这里有类似问题的几个答案，但他们使用 typedef 并在定义的开头添加 std :: ，但我真的不知道为什么。

There's a few answers to similar questions here, but they make use of typedef and adding std:: to the beginning of the definition but I'm really not sure why.

int ComputerPlayer::getBestMoves(){
    //will return the pit number of the best possible move. 

    //map to hold pit numbers and rankings for each possible pit number.
    //map<pitNumber, rank> only adds pit numbers to map if they have seeds in them.

    std::map<int, int> possiblePits; //map
    std::map<int, int>::iterator it; //iterator for map
    for(int index = 1; index <= getBoardSize(); index++){
        if(_board.getPitValue(index) > 0){
            possiblePits.insert( pair<int, int>(index, 0) ); 
        }
    }

    int tempBoardSize = _board.getBoardSize();

    //loop that will analyze all possible pits in the map
    for(it = possiblePits.begin(); it != possiblePits.end(); it++){
        Board tempBoard = _board;
        int pitNum = it->first; 

        int score = analyzePlay(pitNum, tempBoard, possiblePits);
    }
    return 0; 
}

int analyzePlay(int pitNum, Board tempBoard, std::map<int, int> &possibleMoves){
    int tempBoardSize = tempBoard.getBoardSize(); 
    int tempSeeds = tempBoard.getPitValue(pitNum);
    int lastPitSown; 

    tempBoard.setPitToZero(pitNum); 

    for(int index = 1; index <= tempSeeds; index++){

        if(pitNum == tempBoardSize * 2 + 1){
            //skips over human's score pit 
            pitNum += 2; 
            lastPitSown = pitNum;
            tempBoard.incrementPit(pitNum);
        }
        else{
            pitNum++;
            lastPitSown = pitNum;
            tempBoard.incrementPit(pitNum);
        }
    }

    if(tempBoard.getPitValue(lastPitSown) == 1 && lastPitSown >= tempBoardSize + 2 && lastPitSown <= tempBoardSize * 2 + 1){
        //turn ends. last seed sown into empty pit on opponent side. 

    }
    else if(tempBoard.getPitValue(lastPitSown) > 1 && lastPitSown != tempBoardSize + 1){
        //keep playing with next pit. last seed was sown into non-empty pit. 

    }
    else if(lastPitSown == tempBoardSize + 1){
        //extra turn. last seed sown into score pit.

    }
    else if(tempBoard.getPitValue(lastPitSown) == 1 && lastPitSown != tempBoardSize + 1 && lastPitSown <= tempBoardSize && lastPitSown >= 1 ){
        //turn ends. last seed sown into empty pit on your side. capture.


    }
    return 0;
}

我得到的错误：

Error   1   error LNK2019: unresolved external symbol "public: int __thiscall ComputerPlayer::analyzePlay(int,class Board,class std::map<int,int,struct std::less<int>,class std::allocator<struct std::pair<int const ,int> > > &)" (?analyzePlay@ComputerPlayer@@QAEHHVBoard@@AAV?$map@HHU?$less@H@std@@V?$allocator@U?$pair@$$CBHH@std@@@2@@std@@@Z) referenced in function "public: int __thiscallComputerPlayer::getBestMoves(void)" (?getBestMoves@ComputerPlayer@@QAEHXZ)    C:\Users\Josh\Dropbox\Congkak_2\Congkak_2\ComputerPlayer.obj
Error   2   error LNK1120: 1 unresolved externals   C:\Users\Josh\Dropbox\Congkak_2\Debug\Congkak_2.exe

推荐答案

两件事：

• 在顶部添加＃include< map> 并使用 std :: map 而不是 map 。


• code> function2 上面 function1 或至少声明 function2 $ c> function1 。

• Add #include<map> at the top, and use std::map instead of just map.
• Define function2 above function1 Or at least declare function2 above function1.

以下是两者的完成方式：

Here is how both should be done:

#include<map>

void function2(std::map<int, int> &temp_map); //forward declaration

void function1(){
    std::map<int, int>  my_map; //automatic variable 
                                //no need to make it pointer!
    function2(my_map); 
}

void function2(std::map<int, int> &temp_map){
    //do stuff with the map
}

还要注意尽可能避免 new 默认情况下使用自动变量，除非你有很强的理由不使用它。

Also note that avoid new as much as possible. Use automatic variables by default, unless you've very strong reason not to use it.

自动变量速度快，代码看起来整洁干净。使用它们更容易编写异常安全代码。

Automatic variables are fast, and the code looks neat and clean. With them it is easier to write exception-safe code.

编辑：

，你也意识到，

我忘了添加该函数是其开头的一部分的类。如：Player :: function2（std :: map & temp_map）{}

I forgot to add the Class that the function was part of to the beginning of it. as in: Player::function2(std::map<int, int> &temp_map){}

在评论中。

很好，你自己想出来了。但仍然，总是在你的第一篇文章，当你提出问题时发布错误。记住这一点。

Good that you figured it out yourself. But still, always post the error in your very first post, when you ask the question. Remember this.

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