Why do we don't have hash and pred functors for a map?

Question

In case of unordered_map we define the hash and pred functors whenever we are using user-defined keys.

The template syntax for a map is as follows:

template < class Key,                                     // map::key_type
       class T,                                           // map::mapped_type
       class Compare = less<Key>,                         // map::key_compare
       class Alloc = allocator<pair<const Key,T> >       // map::allocator_type
       > class map;

In case of map there is no hash and pred functors option. Do we never have collisions in case of map. If collisions happen then why don't we have the hash and pred functors as in unordered_map? Am I missing something here?

[`std::map`](https://en.cppreference.com/w/cpp/container/map) (as well as [`std::set`](https://en.cppreference.com/w/cpp/container/set)) is usually implemented as a *tree*. With a tree there are no collisions. — Some programmer dude, Sep 26 '19 at 12:20

score 5 · Answer 1 · answered Sep 26 '19 at 12:21

5

std::map is not a hash table and thus doesn't use a hash function. Instead, it requires operator< for ordering the values contained in the map.

answered Sep 26 '19 at 12:21

Violet Giraffe

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score 5 · Accepted Answer · answered Sep 26 '19 at 12:25

std::map and std::unordered_map are two different types of containers that both provided key-value pair mapping. How they do that though is completely different.

std::map uses a tree structure for its implementation. Typically this is an RBTree but any tree that can guarantee worst case O(logN) operations will work. This means it only needs to have a comparison operator for the key type since you can get total ordering and check for equality with a comparator that implements a strict weak ordering. This means you'll never have a hash collision since you aren't using a hash.

std::unordered_map is based on a hash table implementation. Since it hashes the key, you need a hash operator. You also need a comparison operator since two values could hash to the same value (hash collision). Without the comparison operator you would not be able to tell if the duplicate hash is really a duplicate item.

Why do we don't have hash and pred functors for a map?

2 Answers2