multimap on C-style string key fails to insert entries

Question

I am trying to create a multimap indexed on a C-style string, as shown in the following code segment:

    #include <cstring>
    #include <map>
    #include <iostream>

    using namespace std;

    int main(void)
    {
        int i, j;
        int (*fn_pt)(const char *, const char *) = strcmp;
        multimap<char *, char *, int (*)(const char *, const char *)>a(fn_pt);

        for (i = 0; i < 2; i++)
        {
            char key[2];
            sprintf(key, "%d", i);
            for (j = 0; j< 5; j++)
            {
                char value[2];
                sprintf(value, "%d", j);
                a.insert(pair<char *, char *>(key, value));
            }
        }

        for (i = 0; i < 2; i++)
        {
            char key[2];
            sprintf(key, "%d", i);
            multimap<char *, char *>::iterator it = a.find(key);
            while (it != a.end())
            {
                cout << it->first << "\t" << it->second <<endl;
                it++;
            }
        }
    }

Simply changing the key in the above program to integer gives me the expected result. But, indexing the multimap on a string is giving me something unexpected (only rows of 1's and 4's separated by space), instead of showing me every value for every key value used.

Where am I going wrong in this?

Thanks

Answer 1

strcmp is a wrong predicate to use in multimap. The predicate shall satisfy the following:

The expression comp(a,b), where comp is an object of this comparison class and a and b are key values, shall return true if a is to be placed at an earlier position than b in a strict weak ordering operation.

strcmp violates that because it returns a nonzero value if the strings are unequal, either a < b or a > b.

You should define your own predicate which returns true if and only if the first string is less than the second.