Question

有没有办法从STL hash_map获取所有密钥？或者我必须使用set或hash_set之类的东西在插入之前记录它们？

Answer 1

hash_map<string, void *> hashMap;

vector<string> keys;
keys.reserve(hashMap.size());

for (hash_map<string, void *>::iterator iter = hashMap.begin(); 
                                        iter != hashMap.end(); 
                                        ++iter)
{
    keys.push_back(iter->first);
}

Answer 2

简单地遍历hash_map;对于每次迭代，iter->first是关键。

Answer 3

以Igor Oks的回答为基础：

hash_map<string, void *> hashMap;

vector<string> keys;
keys.reserve(hashMap.size());

transform(hashMap.begin(), hashMap.end(), back_inserter(keys),
    select1st<hash_map<string, void*>::value_type>());

Answer 4

您可能想要遍历hash_map，并从当前迭代器值指向的对中提取第一个元素（该对的第一个元素实际上是一个键）。

// Assuming that hm is an instance of hash_map:
for (auto it = hm.begin(); it != hm.end(); ++it) // for each item in the hash map:
{
    // it->first is current key
    // ... you can push_back it to a vector<Key> or do whatever you want
}

这是一个将hash_map中的键提取到vector的函数：

template <typename Key, typename Type, typename Traits, typename Allocator>
vector<Key> extract_keys(const hash_map<Key, Type, Traits, Allocator> & hm)
{
    vector<Key> keys;

    // If C++11 'auto' is not available in your C++ compiler, use:
    // 
    //   typename hash_map<Key, Type, Traits, Allocator>::const_iterator it;
    //   for (it = hm.begin(); ...)
    //
    for (auto it = hm.begin(); it != hm.end(); ++it)
    {
        keys.push_back(it->first);
    }

    return keys;        
}

如何从STL hash_map获取所有密钥？

4 个答案: