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<unordered_map>

C++11
unordered_map
C++11
unordered_multimap

unordered_map

C++11
unordered_map::~unordered_map
C++11
unordered_map::unordered_map
member functions
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  unordered_map::at
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  unordered_map::begin
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  unordered_map::bucket
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  unordered_map::bucket_count
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  unordered_map::bucket_size
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  unordered_map::cbegin
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  unordered_map::cend
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  unordered_map::clear
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  unordered_map::count
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  unordered_map::emplace
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  unordered_map::emplace_hint
- C++11
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  unordered_map::end
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  unordered_map::equal_range
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  unordered_map::erase
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  unordered_map::get_allocator
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  unordered_map::hash_function
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  unordered_map::insert
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  unordered_map::key_eq
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  unordered_map::load_factor
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  unordered_map::max_bucket_count
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  unordered_map::max_load_factor
- C++11
  unordered_map::max_size
- C++11
  unordered_map::operator[]
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  unordered_map::operator=
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  unordered_map::rehash
- C++11
  unordered_map::reserve
- C++11
  unordered_map::size
- C++11
  unordered_map::swap
non-member overloads
- C++11
  operators (unordered_map)
- C++11
  swap (unordered_map)

public member function

<unordered_map>

std::unordered_map::reserve

void reserve ( size_type n );

Request a capacity change

Sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least n elements.

If n is greater than the current bucket_count multiplied by the max_load_factor, the container's bucket_count is increased and a rehash is forced.

If n is lower than that, the function may have no effect.

Parameters

n: The number of elements requested as minimum capacity.
Member type size_type is an unsigned integral type

Return value

none

Example

// unordered_map::reserve
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_map<std::string,std::string> mymap;

  mymap.reserve(6);

  mymap["house"] = "maison";
  mymap["apple"] = "pomme";
  mymap["tree"] = "arbre";
  mymap["book"] = "livre";
  mymap["door"] = "porte";
  mymap["grapefruit"] = "pamplemousse";

  for (auto& x: mymap) {
    std::cout << x.first << ": " << x.second << std::endl;
  }

  return 0;
}

Possible output:

book: livre
house: maison
grapefruit: pamplemousse
tree: arbre
apple: pomme
door: porte

By calling reserve with the size we expected for the unordered_map container we avoided the multiple rehashes that the increases in container size could have produced and optimized the size of the hash table.

Complexity

In case of rehash,
Average case: linear in container size.
Worst case: quadratic in container size.

Iterator validity

If a rehash happens, all iterators are invalidated, but references and pointers to individual elements remain valid.
If no actual rehash happens, no changes.

See also

unordered_map::rehash: Set number of buckets (public member function)

unordered_map::bucket_count: Return number of buckets (public member function)

unordered_map::max_load_factor: Get or set maximum load factor (public member function)

unordered_map::size: Return container size (public member function)

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