c ++无法通过指向对象的指针分配对象的指针

Question

我无法通过将指针分配给指向该对象的另一个指针来分配该对象的指针。 这是我的代码：

在Map.h中：

using namespace std;
class Map;
Map *CurrentMap;
class Map
{
    private:
        Map *NM, *EM, *SM, *WM;
        string NMe, EMe, SMe, WMe;
    public:
        void Initialize(Map N, Map E, Map S, Map W, string MeN, string MeE, string MeS, string MeW)
        {
            NM = &N; EM = &E; SM = &S; WM = &W;
            NMe = MeN; EMe = MeE; SMe = MeS; WMe = MeW;
        }

        void GoNorth()
        {
            cout << NMe << endl;
            CurrentMap = NM;  //This is the problem, this is where it crashes my program
        }

        void GoEast()
        {
            cout << EMe << endl;
            CurrentMap = EM;
        }

        void GoSouth()
        {
            cout << SMe << endl;
            CurrentMap = SM;
        }

        void GoWest()
        {
            cout << WMe << endl;
            CurrentMap = WM;
        }
};

在Main.cpp中：

#include <iostream>
#include <string>
#include "Map.h"
using namespace std;

int main()
{
    Map A,B,C,D,E,F;
    A.Initialize(B,C,D,E,"Towards B","Towards C","Towards D","Towards E");
    B.Initialize(D,F,A,F,"Towards D","Towards F","Towards A","Towards F");
    C.Initialize(F,E,F,A,"Towards F","Towards E","Towards F","Towards A");
    D.Initialize(A,F,B,F,"Towards A","Towards F","Towards B","Towards F");
    E.Initialize(F,A,F,C,"Towards F","Towards A","Towards F","Towards C");
    F.Initialize(A,A,A,A,"Towards A","Towards A","Towards A","Towards A");
    CurrentMap = &A;

    CurrentMap->GoNorth();
    CurrentMap->GoNorth();
    CurrentMap->GoNorth();
    system("pause");
    return 0;
}

上面的代码的预期输出是：

    Towards B
    Towards D
    Towards A

但是我得到的是：

    Towards B
    //Crash, Unhandled Exception: Access violation reading location 0xcccccccc 

Answer 1

您按值传递，并使用这些值创建临时副本并获取它们的地址，一旦它们超出范围，这当然将是无效的。

更改初始化的签名以获取指针或引用。

`void Initialize(Map *N, Map *E, Map *S, Map *W, string MeN, string MeE, string MeS, string MeW);`

并在初始化时获取地址。

`A.Initialize(&B, &C, &D, &E,"Towards B","Towards C","Towards D","Towards E");`

Answer 2

在下面的代码中，您正在将指针分配给按值传递的参数，这意味着，一旦Initialize（）返回，则类的指针现在指向不再存在的对象。

    void Initialize(Map N, Map E, Map S, Map W, string MeN, string MeE, string MeS, string MeW)
    {
        NM = &N; EM = &E; SM = &S; WM = &W;
        NMe = MeN; EMe = MeE; SMe = MeS; WMe = MeW;
    }

如果您确实希望您的类保留指向您在main（）中声明的对象的指针，则需要进行按引用传递：

    void Initialize(Map & N, Map & E, Map & S, Map & W, string MeN, string MeE, string MeS, string MeW)
    {
        NM = &N; EM = &E; SM = &S; WM = &W;
        NMe = MeN; EMe = MeE; SMe = MeS; WMe = MeW;
    }

...或者您可以更改Initialize（）来接受指针参数（例如Map *）。 但是您可能想重新考虑设计，只保留对象的副本，而不是指向在其他地方声明的对象的指针。 这样可以减少脆弱性，因此您不太可能犯导致程序崩溃的错误。