std::unordered_multimap 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 ).

• (6) Inserts elements from initializer list ilist.

• (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

• (1-4) - Returns an iterator to the inserted element.
• (5-6) - (none)
• (7-8) - End iterator if nh was empty, iterator pointing to the inserted element otherwise.

Complexity

• (1-4)
  Average case, constant - O(1).
  Worst case: O(size()).

• (5-6)
  Average case, linear in the number of elements to insert - O(N), where N is the number of elements to insert.
  Worst case - O(N * size() + N).

• (7-8)
  Average case, constant - O(1).
  Worst case, linear in size of the container - 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_multimap<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"}});

	std::cout << "contents:\n";
	for(auto& p: dict)
			std::cout << " " << p.first << " => " << p.second << '\n';
}

Possible output

contents:
 5 => five
 2 => two
 1 => one
 3 => three
 4 => another four
 4 => four