std::unordered_map insert() method

since C++17

// (1) Non const version only
std::pair<iterator,bool> insert( value_type&& value );

// (2) Non const version only
template< class P >
std::pair<iterator,bool> insert( P&& value );

// (3) Non const version only
iterator insert( const_iterator hint, value_type&& value );

// (4) Non const version only
template< class P >
iterator insert( const_iterator hint, P&& value );

// (5) Non const version only
template< class InputIt >
void insert( InputIt first, InputIt last );

// (6) Non const version only
void insert( std::initializer_list<value_type> ilist );

// (7) Non const version only
insert_return_type insert( node_type&& nh );

// (8) Non const version only
iterator insert( const_iterator hint, node_type&& nh );

until C++17

// (1) Non const version only
std::pair<iterator,bool> insert( const value_type& value );

// (2) Non const version only
template< class P >
std::pair<iterator,bool> insert( P&& value );

// (3) Non const version only
iterator insert( const_iterator hint, const value_type& value );

// (4) Non const version only
template< class P >
iterator insert( const_iterator hint, P&& value );

// (5) Non const version only
template< class InputIt >
void insert( InputIt first, InputIt last );

// (6) Non const version only
void insert( std::initializer_list<value_type> ilist );

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 ).

  If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

• (6) Inserts elements from initializer list ilist.

  If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

• (7) If nh is an empty node handle, does nothing.

  Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key().

Undefined behaviour

The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().

• (8) If nh is an empty node handle, does nothing and returns the end iterator.

  Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key(), and returns the iterator pointing to the element with key equivalent to nh.key() (regardless of whether the insert succeeded or failed).

  If the insertion succeeds, nh is moved from, otherwise it retains ownership of the element. The element is inserted as close as possible to hint.

Undefined behaviour

The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().

since C++17

Invalidation

If the insertion is successful, pointers and references to the element obtained while it is held in the node handle are invalidated and pointers and references obtained to that element before it was extracted become valid.

until C++17

Invalidation

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
• nh - a compatible node handle

Type requirements

• (5) InputIt must meet the requirements of LegacyInputIterator.

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 (true if insertion happened, false if it did not).
• (3-4) Returns an iterator to the inserted element, or to the element that prevented the insertion.
• (5-6) (none)
• (7) Returns an insert_return_type with the members initialized as follows:
  • If nh is empty, inserted is false, position is end(), and node is empty.
  • Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty.
  • If the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().
• (8) End iterator if nh was empty, iterator pointing to the inserted element if insertion took place, and iterator pointing to an element with a key equivalent to nh.key() if it failed.

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)
  Worst case: O(N*size()+N)

• (7-8)
  Average case: O(1).
  Worst case: O(size()).

Exceptions

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

important

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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

Main.cpp

#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