如何從STL hash_map獲取所有密鑰？

Question

有沒有辦法從STL hash_map獲取所有密鑰？或者我必須使用像set或hash_set之類的東西在插入之前記錄它們？

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

簡單地遍歷hash_map;對於每次迭代，iter->first是關鍵。

Answer 3

大廈伊戈爾OKS'回答：

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

您可能希望通過的hash_map迭代，並提取一對由當前迭代器價值指向的第一個元素（一對的第一個元素是事實關鍵）。

// 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 
} 

這是一個可能的功能來提取一個的hash_map鍵向量：

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;   
}