hash_map give unexpected value.

[eagerlearner]

Can anyone check this out, why does this give me the default value of 0 instead of 1 ? Thank you.

#include <iostream>
#include <hash_map>
#include <string>
using namespace std;

int main()
{
	stdext::hash_map<const char *, int> hm;
	hm["abc"] = 1;
	hm["def"] = 2;

	string s = "abc";

	cout << hm[s.c_str()];

	return 0;
}

[JosAH]

Can anyone check this out, why does this give me the default value of 0 instead of 1 ? Thank you.

#include <iostream>
#include <hash_map>
#include <string>
using namespace std;

int main()
{
	stdext::hash_map<const char *, int> hm;
	hm["abc"] = 1;
	hm["def"] = 2;

	string s = "abc";

	cout << hm[s.c_str()];

	return 0;
}

If you don't supply a map with a comparator, the default < (less) will be used.
Comparing (character) pointers using the < operator doesn't work so you have
to supply the map an explicit comparator:

[code=cpp]
struct comp {
bool operator() (const char* lhs, const char* rhs) const {

return strcmp(lhs, rhs);
}
};

int main() {
map<const char *, int, comp> hm;
...
[/code]

I used an ordinary map instead of your hash_map, but I don't think it matters.
(the hash_map is not in my version of the STL).

kind regards,

Jos

[eagerlearner]

Thanks for the reply, I just found out that for hash_map it has to derive the class from hash_compare and at least both the hash function and comparison must be specified. Thank you.

class hasher : public stdext::hash_compare<const char *>
{
public:

size_t operator() (const char* s) const
{
	size_t h = 0;
	size_t max, i;
	for(i = 0, max = strlen(s); i < max; i++)
	{
	  h = 31 * h + s[i];
	}
	return h;
}
	bool operator() (const char* lhs, const char* rhs) const
	{
		return (strcmp(lhs, rhs) < 0);
	}
};

static stdext::hash_map<const char *, int, hasher> hm;