Basics of Hash Tables
The containers are made up of a number of buckets, each of which can contain
any number of elements. For example, the following diagram shows a boost::unordered_set
with 7 buckets containing 5 elements, A
,
B
, C
, D
and E
(this is just for illustration, containers will typically
have more buckets).

In order to decide which bucket to place an element in, the container applies
the hash function, Hash
, to the element’s key (for sets the key is the whole element, but is referred to as the key
so that the same terminology can be used for sets and maps). This returns a
value of type std::size_t
. std::size_t
has a much greater range of values
then the number of buckets, so the container applies another transformation to
that value to choose a bucket to place the element in.
Retrieving the elements for a given key is simple. The same process is applied
to the key to find the correct bucket. Then the key is compared with the
elements in the bucket to find any elements that match (using the equality
predicate Pred
). If the hash function has worked well the elements will be
evenly distributed amongst the buckets so only a small number of elements will
need to be examined.
You can see in the diagram that A
& D
have been placed in the same bucket.
When looking for elements in this bucket up to 2 comparisons are made, making
the search slower. This is known as a collision. To keep things fast we try to
keep collisions to a minimum.
If instead of boost::unordered_set
we had used boost::unordered_flat_set
, the
diagram would look as follows:

In open-addressing containers, buckets can hold at most one element; if a collision happens
(like is the case of D
in the example), the element uses some other available bucket in
the vicinity of the original position. Given this simpler scenario, Boost.Unordered
open-addressing containers offer a very limited API for accessing buckets.
All containers |
|
---|---|
Method |
Description |
|
The number of buckets. |
Closed-addressing containers only |
|
Method |
Description |
|
An upper bound on the number of buckets. |
|
The number of elements in bucket |
|
Returns the index of the bucket which would contain |
|
Return begin and end iterators for bucket |
|
|
|
|
|
|
|
|
|
Controlling the Number of Buckets
As more elements are added to an unordered associative container, the number
of collisions will increase causing performance to degrade.
To combat this the containers increase the bucket count as elements are inserted.
You can also tell the container to change the bucket count (if required) by
calling rehash
.
The standard leaves a lot of freedom to the implementer to decide how the number of buckets is chosen, but it does make some requirements based on the container’s load factor, the number of elements divided by the number of buckets. Containers also have a maximum load factor which they should try to keep the load factor below.
You can’t control the bucket count directly but there are two ways to influence it:
-
Specify the minimum number of buckets when constructing a container or when calling
rehash
. -
Suggest a maximum load factor by calling
max_load_factor
.
max_load_factor
doesn’t let you set the maximum load factor yourself, it just
lets you give a hint. And even then, the standard doesn’t actually
require the container to pay much attention to this value. The only time the
load factor is required to be less than the maximum is following a call to
rehash
. But most implementations will try to keep the number of elements
below the max load factor, and set the maximum load factor to be the same as
or close to the hint - unless your hint is unreasonably small or large.
All containers |
|
---|---|
Method |
Description |
|
Construct an empty container with at least |
|
Construct an empty container with at least |
|
The average number of elements per bucket. |
|
Returns the current maximum load factor. |
|
Changes the container’s maximum load factor, using |
|
Changes the number of buckets so that there at least |
Open-addressing and concurrent containers only |
|
Method |
Description |
|
Returns the maximum number of allowed elements in the container before rehash. |
A note on max_load
for open-addressing and concurrent containers: the maximum load will be
(max_load_factor() * bucket_count()
) right after rehash
or on container creation, but may
slightly decrease when erasing elements in high-load situations. For instance, if we
have a boost::unordered_flat_map
with size()
almost
at max_load()
level and then erase 1,000 elements, max_load()
may decrease by around a
few dozen elements. This is done internally by Boost.Unordered in order
to keep its performance stable, and must be taken into account when planning for rehash-free insertions.