std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::insert

Overload (3) is equivalent to emplace(std::forward <P>(value)) and only participates in overload resolution if std::is_constructible <value_type, P&&>::value == true.

4-6) Inserts value, using hint as a non-binding suggestion to where the search should start.

Overload (6) 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.

7) 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).

If [first, last) is not a valid range, or first and/or last are iterators into *this, the behavior is undefined.

8) 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).

9) 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().

10) 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. hint is used as a non-binding suggestion to where the search should start. The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated. 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)

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().

10) 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.

[edit] Exceptions

1-6) If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

7,8) No exception safety guarantee.

9,10) If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

[edit] Complexity

1-6) Average case: O(1), worst case O(size()).

7,8) Average case: O(N), where N is the number of elements to insert. Worst case: O(N * size() + N).

9,10) Average case: O(1), worst case O(size()).

[edit] Notes

The hinted insert ((4-6) and (10)) 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.

[edit] Example

Run this code

#include <iostream>
#include <string>
#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"}});
 
 const 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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2005	C++11	overloads (3) and (6) would only participate in overload resolution if `P` is implicitly convertible to `value_type`	only participates if `value_type` is constructible from P&&