Linux包含hash_map和hash_set的not declared问题

当在Linux下cpp文件包含hash_map或hash_set时，会出现"‘hash_map’ was not declared in this scope"问题。

#include 
#include 
#include 
using namespace std;

int main(void)
{
	hash_map hmap;

	hmap[1] = "hi hdu1";
	hmap[2] = "hi hdu2";
	hmap[3] = "hi hdu3";

	hash_map::iterator iter;
	iter = hmap.find(2);
	if (iter != hmap.end())
	{
		cout << iter->second << endl;
	}
	else
	{
		cout << "not find" << endl;
	}

	return 0;
}

原因：hash_map是声明在__gnu_cxx命名空间的，所以只要在程序中添加using namespace __gnc_cxx就可以了^_^

#include 
#include 
#include 
using namespace std;
using namespace __gnu_cxx;

int main(void)
{
	hash_map hmap;

	hmap[1] = "hi hdu1";
	hmap[2] = "hi hdu2";
	hmap[3] = "hi hdu3";

	hash_map::iterator iter;
	iter = hmap.find(2);
	if (iter != hmap.end())
	{
		cout << iter->second << endl;
	}
	else
	{
		cout << "not find" << endl;
	}

	return 0;
}