std::unordered_map::insert

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

 

std::unordered_map

Member functions

unordered_map::unordered_map

unordered_map::~unordered_map

unordered_map::operator=

unordered_map::get_allocator

Iterators

unordered_map::beginunordered_map::cbegin

unordered_map::endunordered_map::cend

Capacity

unordered_map::erase

unordered_map::size

unordered_map::max_size

Modifiers

unordered_map::clear

unordered_map::insert

unordered_map::insert_or_assign (C++17)

unordered_map::emplace

unordered_map::emplace_hint

unordered_map::try_emplace (C++17)

unordered_map::erase

unordered_map::swap

Lookup

unordered_map::count

unordered_map::find

unordered_map::equal_range

Bucket interface

unordered_map::begin2

unordered_map::end2

unordered_map::bucket_count

unordered_map::max_bucket_count

unordered_map::bucket_size

unordered_map::bucket

Hash policy

unordered_map::load_factor

unordered_map::max_load_factor

unordered_map::rehash

unordered_map::reserve

Observers

unordered_map::hash_function

unordered_map::key_eq

 

std::pair<iterator,bool> insert( const value_type& value );	(1)	(since C++11)
template< class P > std::pair<iterator,bool> insert( P&& value );	(2)	(since C++11)
std::pair<iterator,bool> insert( value_type&& value );	(2)	(since C++17)
iterator insert( const_iterator hint, const value_type& value );	(3)	(since C++11)
template< class P > iterator insert( const_iterator hint, P&& value );	(4)	(since C++11)
iterator insert( const_iterator hint, value_type&& value );	(4)	(since C++17)
template< class InputIt > void insert( InputIt first, InputIt last );	(5)	(since C++11)
void insert( std::initializer_list<value_type> ilist );	(6)	(since C++11)

Inserts element(s) into the container, if the container doesn't already contain an element with an equivalent key.

1-2) Inserts value. The overload (2) is equivalent to emplace(std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.

3-4) Inserts value, using hint as a non-binding suggestion to where the search should start. The overload (4) is equivalent to emplace_hint(hint, std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.

5) Inserts elements from range [first, last).

6) Inserts elements from initializer list ilist.

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor()*bucket_count().

Parameters

hint	-	iterator, used as a suggestion as to where to insert the content
value	-	element value to insert
first, last	-	range of elements to insert
ilist	-	initializer list to insert the values from
Type requirements
- InputIt must meet the requirements of InputIterator.

Return value

1-2) Returns a pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool denoting whether the insertion took place.

3-4) Returns an iterator to the inserted element, or to the element that prevented the insertion.

5-6) (none)

Exceptions

1-4) If an exception is thrown by any operation, the insertion has no effect.

Complexity

1-4) Average case: O(1), worst case O(size())

5-6) Average case: O(N), where N is the number of elements to insert. Worse case: O(N*size()+N)

Notes

The hinted insert (3,4) does not return a boolean in order to be signature-compatible with positional insert on sequential containers, such as std::vector::insert. This makes it possible to create generic inserters such as std::inserter. One way to check success of a hinted insert is to compare size() before and after.

Example

Run this code

#include <string>
#include <iostream>
#include <unordered_map>
 
int main ()
{
    std::unordered_map<int, std::string> dict = {{1, "one"}, {2, "two"}};
    dict.insert({3, "three"});
    dict.insert(std::make_pair(4, "four"));
    dict.insert({{4, "another four"}, {5, "five"}});
 
    bool ok = dict.insert({1, "another one"}).second;
    std::cout << "inserting 1 -> \"another one\" " 
              << (ok ? "succeeded" : "failed") << '\n';
 
    std::cout << "contents:\n";
    for(auto& p: dict)
        std::cout << " " << p.first << " => " << p.second << '\n';
}

Possible output:

inserting 1 -> "another one" failed
contents:
 5 => five
 1 => one
 2 => two
 3 => three
 4 => four

See also

emplace	constructs element in-place (public member function)
emplace_hint	constructs elements in-place using a hint (public member function)