如何使用字符串字符串hash_map（hash_map <string，string，stringHashFunction>在Linux C++

Question

我定義了一個hash_map <在GCC中的字符串，字符串，stringHashFunction> stringHashMap我保證stringHashFunction是正確的，因爲我可以在正確的hash_map

當我打電話

string a = "sgsg"; 
string temp = stringHash[a]; 

編譯器bug報告使用字符串到：

error: passing ‘const __gnu_cxx::hash_map, std::basic_string, StringHashFunctionStruct>’ as ‘this’ argument of ‘_Tp& __gnu_cxx::hash_map<_Key, _Tp, _HashFn, _EqualKey, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string, _Tp = std::basic_string, _HashFn = StringHashFunctionStruct, _EqualKey = std::equal_to >, _Alloc = std::allocator >, __gnu_cxx::hash_map<_Key, _Tp, _HashFn, _EqualKey, _Alloc>::key_type = std::basic_string]’ discards qualifiers [-fpermissive]

爲什麼這可能發生？我應該如何使用字符串來字符串hashMap？

Answer 1

[]訪問者可以修改地圖，因此它不能是非常量。使用找到代替

從http://www.cplusplus.com/reference/stl/map/operator%5B%5D/

Notice how the last access (to element 'd') inserts a new element in the map with that key and initialized to its default value (an empty string) even though it is accessed only to retrieve its value. Member function map::find does not produce this effect.

Answer 2

stringHash是直接const或傳遞/存儲爲const引用。 operator[]是一個非const函數，因爲它可能需要插入一個項目。如果您需要在const哈希容器中查找項目，只需使用find方法。