Re: Some strange behaviors of hash_map

[Jeff Schwab]

杨 波 wrote:

>    I am trying to use the ext/hash_map of gnu C++, but I observe some
> strange behaviors, please help me, my code is :


> bool SE::operator ()(const char *a, const char *b)
> {
>         return strcmp(a, b);
> }

I think you meant:
return strcmp(a, b) == 0;

>         hash_map<const char *, int, hash<const char *>, SE> map;
>         map["abc"] = 1;
>         map["ab"] = 10;
>
>         cout << map["abc"] << endl << map["ab"] << endl << map["d"] <<
> endl;;

> But to my surprise, all the three map["..."] output is '0'

In C++, there is rarely a good reason to represent strings as char 
const*.  Things will go a lot more smoothly if you just stick with a 
string class, and use its c_str member function as needed.  For example:

/* GNU */
#include <ext/hash_map>

/* C++ Standard */
#include <iostream>
#include <string>

namespace gnu = __gnu_cxx;

struct string_hash
{
    std::size_t operator()(std::string const& s) const {
        return gnu::hash<char const*>( )(s.c_str());
    }
};

int main() {

    gnu::hash_map<std::string, int, string_hash> map;

    map["abc"] = 1;
    map["ab"] = 10;

    std::cout << map["abc"] << '\n';
    std::cout << map["ab"] << '\n';
    std::cout << map["d"] << '\n';

    return 0;
}