GCC中使用hash_map

If you have tried to use some STL containers with GCC, such as hash_map:

// error: hash_map: No such file or directory
#include <hash_map>

int main()
{

// error: ‘hash_map’ is not a member of ‘std’
std::hash_map<int,int> hm;

return 0;
}
Then you have realized that the code above does not compile. That's because on GCC, hash_map is not regarded as a standard container, but rather as a extension included in the __gnu_cxx namespace. In order to use hash_map and other extended containers with a minimum impact in your code (which is very important if it's intended to be cross-platform), you can use the following solution:
#ifdef __GNUC__
#include <ext/hash_map>
#else
#include <hash_map>
#endif

namespace std
{
using namespace __gnu_cxx;
}

int main()
{

std::hash_map<int,int> hm;

return 0;
}

Hope that helps.