How to allocate map key using malloc?

Question

If I try the code below, it stores the address as a key and not the value, therefore "the same key is stored twice"

static map<const char *, int> lMap;
    const char * msg = "hhhhh";

    char *buf = (char *) malloc(6);
    strcpy(buf, msg);

    lMap.insert(make_pair(buf, 85));
    buf = (char *) calloc(5, sizeof (char));

    strcpy(buf, msg);
    lMap.insert(make_pair(msg, 85));

    cout << "size: " << lMap.size() << endl;
    map<const char *, int>::const_iterator it2;
    for (it2 = lMap.begin(); it2 != lMap.end(); ++it2) {
        cout << it2->first << " | " << it2->second << endl;
    }

printed result:

size: 2
hhhhh | 85
hhhhh | 85

How many characters is the string `"hhhhh"`? ***Including*** the terminator? — Some programmer dude, Jun 27 '16 at 16:06
`malloc` has nothing to do with the issue. Your keys are pointers. Think about what it means to compare pointers. — juanchopanza, Jun 27 '16 at 16:06
if you choose to use pointers as key you get pointers as keys and of course they might point to the same value. So why do you do this? Just use values as keys to get correct behaviour — 463035818_is_not_an_ai, Jun 27 '16 at 16:06
one of the classes in the code is printing the map values on its destructor, but with std::string seems like the value was destroyed — 15412s, Jun 27 '16 at 16:07
The problem you have with `std::string` is something completely different, and warrants a new and different question. — Some programmer dude, Jun 27 '16 at 16:11

score 2 · Answer 1 · answered Jun 27 '16 at 16:07

2

You don't. Use std::string as a key instead.

Unless you are providing an appropriate comparator functor to handleconst char* keys properly, you will get unexpected results anyways.

answered Jun 27 '16 at 16:07

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one of the classes in the code is printing the map values on its destructor, but with std::string seems like the value was destroyed and I'm getting core dump – 15412s Jun 27 '16 at 16:10
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@15412s Well, that's a different question, and most probably not related to using `std::string` as key, but some wrong logic in a different part of your program. – πάντα ῥεῖ Jun 27 '16 at 16:12

How to allocate map key using malloc?

1 Answers1