Question

I have this sample code to insert entries to a multimap. I am trying to delete particular entries of a specified key. But this code goes into infinite loop. Can someone help me with this code?

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main()
{
    multimap<string, string> names;
    string n;

    names.insert(pair<string, string>("Z", "F"));
    names.insert(pair<string, string>("Z", "A"));

    names.insert(pair<string, string>("S", "T"));
    names.insert(pair<string, string>("S", "A"));
    names.insert(pair<string, string>("S", "J"));

    names.insert(pair<string, string>("D", "H"));
    names.insert(pair<string, string>("D", "W"));
    names.insert(pair<string, string>("D", "R"));

    multimap<string, string>::iterator p;


    p = names.find("Z");
    if(p != names.end()) { // found a name
        do {
            cout << n << ", " << p->second;
            cout << endl;
            if (p->second.compare("A") == 0) {
                names.erase(p);
                p++;
            } else {
                p++;
            }
        } while (p != names.upper_bound("Z"));
    }
    else{
        cout << "Name not found.\n";
    }

    p = names.find("Z");
    if(p != names.end()) { // found a name
        do {
            cout << n << ", " << p->second;
            cout << endl;
        } while (p != names.upper_bound("Z"));
    }
    else{
        cout << "Name not found.\n";
    }
    return 0;
}

In the above I am looking up using Key value “Z” and want to delete “A”.

Answer 1

multimap::erase invalidates any iterators to the erase elements, so the lines

names.erase(p);
p++;

erases p, thus invalidating it, and then attempt to increment an invalid iterator. You can fix this by copying p to a temporary, incrementing p, and then erasing the temporary iterator.

multimap<string, string>::iterator temp = p;
++p;
names.erase(temp);

Alternatively if you’re using C++11 then multimap::erase returns the next iterator in the container

p = names.erase(p);

---

Edit: the above isn’t actually the source of your infinite loop. In the second loop you don’t increment p, so it goes forever. However it is still something you should fix as it can cause unpredictable and difficult to track down bugs.