Same key(created with malloc) is added twice to map

Question

How it is possible that the same key is added twice in map? shouldn't it override the value in case the value was changed?

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

    char *buf = (char *) malloc(5);
    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

As a side note: to be able to `strcpy` the string "hhhhh" you need 6, not 5 characters (5 for the `'h'`s and one for the terminating `'\0'`). — 6502, Jun 22 '16 at 07:29
[Using char* as a key in std::map](http://stackoverflow.com/questions/4157687/using-char-as-a-key-in-stdmap?rq=1) — songyuanyao, Jun 22 '16 at 07:29

score 2 · Accepted Answer · answered Jun 22 '16 at 07:26

2

const char* is a pointer. Comparing two pointers with < compares the addresses they point to. By default, std::map uses < to compare keys. Which means that two different const char* objects are distinct keys, even if they happen to contain the same data when interpreted as a nul-terminated string.

You're writing C++, not C. If you want a string, use std::string, not a pointer to something which will hopefully be a nul-terminated buffer.

answered Jun 22 '16 at 07:26

Angew is no longer proud of SO

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lMap should contain object from different class, some of those classes are destroyed. if I'll add std::string to lMap from class that was destroyedwill the value of the string be saved on map, or it will be destroyedalso (scope issue?) – 15412s Jun 22 '16 at 07:32
1

@15412s If you type the map as `std::map`, it will store copies of those strings and will their sole owner, so you will not run into any scoping/premature destruction issues. – Angew is no longer proud of SO Jun 22 '16 at 07:34
the following should be good for my case, right? std::string a="abc"; map.insert(make_pair(a,85)); – 15412s Jun 22 '16 at 07:45
@15412s Yes, exactly. – Angew is no longer proud of SO Jun 22 '16 at 07:47

score 1 · Answer 2 · answered Jun 22 '16 at 07:26

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You don't have the same key twice in the map. The key of the map is a char * (i.e. a pointer). You have two different pointer values that happen to point to identical strings.

answered Jun 22 '16 at 07:26

The Dark

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score 1 · Answer 3 · answered Jun 22 '16 at 07:28

1

     static map<const char *, int> lMap;

The above statement maps int to const char* and not int to value pointed by const char*.

The map does not care what is pointed by the keys,it just cares that there are two different keys hence two entries are made.

answered Jun 22 '16 at 07:28

Gaurav Sehgal

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I think that usually, one says that keys are mapped to values. Not the other way round. – eerorika Jun 22 '16 at 08:06

Same key(created with malloc) is added twice to map

3 Answers3