C++ 程序在读取 NULL 引用时崩溃

Question

这是 C++ 中的链接结构。这里的所有变量和对象都应该有定义的地址。但是，执行情况并非如此。

#include <iostream>
#include <string.h>
#include <stdio.h>
using namespace std;
struct LineData {
    int ID;
    char name[33];
    LineData *next;
};

class Line {
    private:
        LineData *pointerHead;
        LineData *pointerToTail;
        int numObjects;
    public:

        Line();
        ~Line();

        void enterLine(int ID, char *data);
        void enterLine(LineData *lindat);
        LineData *exitLine();
        int count();
};

Line::Line() {
    pointerHead = NULL;
    pointerToTail = NULL;
    numObjects = 0;
    /*pointerHead = NULL;
    pointerToTail = NULL;*/
}

/** Puts an item into the line. **/
void Line::enterLine(int ID, char *data) {

    LineData *temp;
    cout << "Inner sanctums called.\n";
    temp = new LineData;
    temp->ID = ID;
    strcpy(temp->name, data);
    temp->next = NULL;
    cout << "Temp created.\n\n";
    //cout << "pointerHead is... ... " << pointerHead << "!\n"; Not going to work!
    /* Insert into the Line */
    if(pointerHead != 0) {  /* Insert as first in this Line/You shall not pass! Program will not let you pass! BOOM! Crash! */
        cout << "If tried.\n";
        pointerToTail->next = temp;
        pointerToTail = temp;
    }
    else /* Insert at Tail of the Line */
    {
        cout << "pointerHead == NULL. What are you going to do about it?\n";
        cout << temp->ID << "\n";
        pointerHead = temp;
        pointerToTail = temp;
        cout << "Inserted into Line!\n";
    }
}

/** ......
    ......
  ..........
  ..........
    ...... **/

正确编译。

int main() {
    Line *GeorgiaCyclone;
    Line *GiletteIce; // Shaved Ice.
    Line *WoodenMagic;
    Line *SteelCityNinja;
    Line *Egbe; // Hawk.

GeorgiaCyclone->enterLine(4, "Preston");
Egbe->enterLine(2, "Felix");


return 181;
}

每行的地址在创建时都设置为 NULL。使用 this 没有区别。这怎么可能发生？如何缓解这种崩溃？

Answer 1

Line *GeorgiaCyclone;

这会创建一个指向 Line 的指针，但不会创建一个 Line 供其指向。您需要为其分配 Line 的地址:

Line *GeorgiaCyclone = new Line

更好的是，您可以完全避免在 main() 中使用指针:

Line GeorgiaCyclone;

您在 main() 中声明的所有其他 Line* 也是如此。