我尝试使用 unordered_map 将 char* 键散列为整数值。在编写自定义函子以散列和比较 char* 之后,无序映射似乎可以工作。但是,我最终注意到散列有时会返回不正确的结果。我创建了一个测试项目来重现错误。下面的代码创建了一个带有 char* 键和自定义函子的 unordered_map。然后它运行 1000 次循环并记录发生的任何哈希错误。我想知道我的仿函数是否有问题,或者问题是否出在 unordered_map 中。任何帮助,将不胜感激。谢谢!
#include <cstdlib>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <tr1/unordered_map>
using namespace std;
//These varaibles are just used for printing the 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' was hashed to: '%lu'\n", hashChar, hashVal);
printf("strcmp('%s','%s')='%d' and KeyEqual='%d'\n", c1, c2, cmpVal, cmpRet);
}
int main(int argc, char** argv) {
// Create the 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 the inital hash value of 'apple' to see what it 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 a random character
srand((unsigned int) time(NULL));
char randomChar = (rand() % 26 + 'a');
// Use the hash 1000x times to see if it works properly.
for (int i = 0; i < 1000; i++) {
// Fill the buffer with 'apple'
bzero(buffer, sizeof (buffer));
strcpy(buffer, "apple");
// Try to get the value for 'apple' and report an error if it equals zero.
if (hash_map[buffer] == 0) {
printf("\n****Error: '%s'=0 ****\n", buffer);
printStatus();
}
// Fill the buffer with a random string.
bzero(buffer, sizeof (buffer));
buffer[0] = randomChar;
buffer[1] = '\0';
// Hash the random string.
// ** Taking this line out removes the error. However, based on the functors
// it should be acceptable to reuse a buffer with different content.
hash_map[buffer];
// Update the random character.
randomChar = (rand() % 26 + 'a');
}
printf("done!\n");
return EXIT_SUCCESS;
}