The std::unordered_map< key , value > is a hash table based key-value (or map) container. In theory, searching a single element (as opposed to a range) in a hash table based data structure is faster than a tree based data structure (e.g std::map). Let's take an example:
struct Key {
std::string m_str;
int m_i;
};
// Define the std::unordered_map
std::unordered_map<Key, std::string> umap;
// Insert a key-value pair
umap[{"One",1}] = "Value-One";
The user defined type (class/struct) Key must meet some requirements in order to be used as a key in std::unordered_map. What are those requirements?
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