C++ boost unordered_map - determine if key exists in container

Question

In boost::unordered_map how do I determine if a key exists in it or not?

boost::unordered_map<vector<int>, MyValueType> my_hash_map;

if (my_hash_map[non-existent key] == NULL)

The above gets compiler error "no match for operator '=='..."

Is the problem that I am using a custom value type or something else?

Answer 1

您可以使用find方法：

if (my_hash_map.find(non-existent key) == my_hash_map.end())

Answer 2

exist() is spelled count() for any associative container :

if (my_hash_map.count(key)) { /*key exist*/ }

if (!my_hash_map.count(key)) { /*key does not exist*/ }