c++11 - C++ unordered_map with char* key produces unexpected behavior -

i attempted use unordered_map hash char* key integer value. after writing custom functors hash , compare char*, unordered map appeared work. however, noticed hash return incorrect results. created test project reproduce error. code below creates unordered_map char* key , custom functors. runs 1000x cycles , records hash errors occurred. wondering if there wrong functors, or if problem lies within unordered_map. appreciated. thanks!

#include <cstdlib> #include <stdio.h> #include <string.h> #include <time.h> #include <tr1/unordered_map>  using namespace std;  //these varaibles used printing status. static const char* c1; static const char* c2; static int cmpret; static int cmpval; static const char* hashchar; static size_t hashval;  // character compare functor. struct cmpchar {    bool operator()(const char* s1, const char* s2) const {     c1 = s1;     c2 = s2;     cmpval = strcmp(s1, s2);     cmpret = (cmpval == 0);     return cmpret;   } };  // hash functor. struct hashchar {    size_t operator()(const char* str) const {     hashchar = str;     size_t hash = 0;     int c;      while (c = *str++)       hash = c + (hash << 6) + (hash << 16) - hash;      hashval = hash;     return hash;   } };  void printstatus() {   printf("'%s' hashed to: '%lu'\n", hashchar, hashval);   printf("strcmp('%s','%s')='%d' , keyequal='%d'\n", c1, c2, cmpval, cmpret); }  int main(int argc, char** argv) {    // create unordered map.   tr1::unordered_map<const char*, int, hashchar, cmpchar > hash_map;   hash_map["apple"] = 1;   hash_map["banana"] = 2;   hash_map["orange"] = 3;    // grab inital hash value of 'apple' see hashes to.   char buffer[256];   bzero(buffer, sizeof (buffer));   strcpy(buffer, "apple");   if (hash_map[buffer] == 1) {     printf("first hash: '%s'=1\n", buffer);   }   printstatus();    // create random character   srand((unsigned int) time(null));   char randomchar = (rand() % 26 + 'a');    // use hash 1000x times see if works properly.   (int = 0; < 1000; i++) {      // fill buffer 'apple'     bzero(buffer, sizeof (buffer));     strcpy(buffer, "apple");      // try value 'apple' , report error if equals zero.     if (hash_map[buffer] == 0) {       printf("\n****error: '%s'=0 ****\n", buffer);       printstatus();     }      // fill buffer random string.     bzero(buffer, sizeof (buffer));     buffer[0] = randomchar;     buffer[1] = '\0';      // hash random string.     // ** taking line out removes error. however, based on functors     // should acceptable reuse buffer different content.     hash_map[buffer];      // update random character.     randomchar = (rand() % 26 + 'a');   }    printf("done!\n");    return exit_success; } 

you must careful when using char* in containers, char* won't copied may hope.

by using operator[] of unordered_map used key in map not string want.

operator[] supposed insert key map, copying invoking default constructor (see reference), in case, copy buffer[0].

so afterwards, method cmpchar have strange behaviour, next bytes it'll read in keys can anything.

you not have such problems if using string objetcs.