std::unordered_map::insert(3)

NAME
std::unordered_map::insert - std::unordered_map::insert

Synopsis
   std::pair<iterator,bool> insert( const value_type& value ); (1) (since C++11)
   std::pair<iterator,bool> insert( value_type&& value ); (1) (since C++17)
   template< class P > (2) (since C++11)
   std::pair<iterator,bool> insert( P&& value );
   iterator insert( const_iterator hint, const value_type& value ); (3) (since C++11)
   iterator insert( const_iterator hint, value_type&& value ); (3) (since C++17)
   template< class P > (4) (since C++11)
   iterator insert( const_iterator hint, P&& value );
   template< class InputIt > (5) (since C++11)
   void insert( InputIt first, InputIt last );
   void insert( std::initializer_list<value_type> ilist ); (6) (since C++11)
   insert_return_type insert( node_type&& nh ); (7) (since C++17)
   iterator insert( const_iterator hint, node_type&& nh ); (8) (since C++17)
   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(). 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. The behavior is undefined if nh is not empty and
   get_allocator() != nh.get_allocator().
   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().
   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.
   (since C++17)

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
   -
   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.

Exceptions
   1-4) If an exception is thrown by any operation, the insertion has no effect.
    This section is incomplete
    Reason: cases 5-6

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())

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
   (C++11) (public member function)
   emplace_hint constructs elements in-place using a hint
   (C++11) (public member function)
   insert_or_assign inserts an element or assigns to the current element if the key
   (C++17) already exists
                    (public member function)
   inserter creates a std::insert_iterator of type inferred from the argument
                    (function template)