0xC0000005: Access violation executing location 0x00000000. When trying to access the insert function of multimap

问题

I was coding for one of the problems in USACO Training Page, and when I tried to debug my code:

#include <iostream>
#include<fstream>
#include<string>
#include<map>
#include<math.h>
using namespace std;
int translate(string &s) {
    int n = s.length();
    int result = 0;
    for (int i = 0; i < s.length(); i++) {
        int n1 = 0;
        if (s.at(i) == 'Q' || s.at(i) == 'Z') {
            return -1;
        }
        switch (s.at(i)) {
        case 'A':
        case 'B':
        case'C':
            result = result + 2 * pow(10, n-1);
            n--;
            break;
        case 'D':
        case 'E':
        case'F':
            result = result + 3 * pow(10, n - 1);
            n--;
            break;
        case 'G':
        case 'H':
        case 'I':
            result = result + 4 * pow(10, n - 1);
            n--;
            break;
        case 'J':
        case 'K':
        case 'L':
            result = result + 5 * pow(10, n - 1);
            n--;
            break;
        case 'M':
        case 'N':
        case 'O':
            result = result + 6 * pow(10, n - 1);
            n--;
            break;
        case 'P':
        case 'R':
        case 'S':
            result = result + 7 * pow(10, n - 1);
            n--;
            break;
        case 'T':
        case 'U':
        case 'V':
            result = result + 8 * pow(10, n - 1);
            n--;
            break;
        case 'W':
        case 'X':
        case 'Y':
            result = result + 9 * pow(10, n - 1);
            n--;
            break;
    }
    }
    return result;
}
bool mycompare(int n, int m) {
    string a, b;
    a = to_string(n);
    b = to_string(m);
    if (a < b) {
        return true;
    }
    else return false;
}
int main() {
    bool(*ptr)(int, int);
    typedef multimap<int, string, bool(*)(int, int)> mmid;
    mmid pairs(ptr);
    string s1;
    ifstream inFile("dict.txt", ios::in | ios::binary);
    while (cin>>s1) {
        int f =translate(s1);
        pairs.insert(mmid::value_type(f, s1));
    }
    int m;
    cin >> m;
    multimap<int, string>::iterator it;
    while (true) {
        it = pairs.find(m);
        if (it != pairs.end()) {
            cout << it->second << endl;
            pairs.erase(it);
        }
        else {
            break;
        }
    }


    return 0;
}

When I try to enter"ANDY HARRY", when it processes to the second entry,

pairs.insert(mmid::value_type(f, s1));

At this line it gives me the code of 0xC0000005: Access violation executing location 0x00000000. What is wrong with my code? And why is it giving me the error message not on the first entry but on the second one? Thanks.

回答1:

I think the issue is in this code:

bool(*ptr)(int, int);
mmid pairs(ptr);

In the first line, you define a function pointer named ptr, but you leave it uninitialized. This means that when you initialize pairs with ptr, you're initializing it with a garbage comparison function pointer.

To fix this, pass in the actual comparison function you want to use. For example:

mmid pairs(mycompare);

Additionally, this code is quite suspicious:

multimap<int, string>::iterator it;
it = pairs.find(m);

Notice that the type of the multimap doesn't match the type of pairs, so there's no a priori reason to suspect this iterator would work correctly. Consider this instead:

mmid::iterator it;

来源：https://stackoverflow.com/questions/38254209/0xc0000005-access-violation-executing-location-0x00000000-when-trying-to-acces