std::unordered_map::insert

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

#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

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