Difference between hash_map and unordered_map?

Since there was no hash table defined in the C++ standard library, different implementors of the standard libraries would provide a non-standard hash table often named hash_map. Because these implementations were not written following a standard they all had subtle differences in functionality and performance guarantees.

Starting with C++11 a hash table implementation has been added to the C++ standard library standard. It was decided to use an alternate name for the class to prevent collisions with these non-standard implementations and to prevent inadvertent use of the new class by developers who had hash_table in their code.

The chosen alternate name is unordered_map which really is more descriptive as it hints at the class’s map interface and the unordered nature of its elements.

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