std::set::insert

std::pair<iterator,bool> insert( const value_type& value );	(1)
std::pair<iterator,bool> insert( value_type&& value );	(2)	(since C++11)
	(3)
iterator insert( iterator hint, const value_type& value );		(until C++11)
iterator insert( const_iterator hint, const value_type& value );		(since C++11)
iterator insert( const_iterator hint, value_type&& value );		(4)	(since C++11)
template< class InputIt > void insert( InputIt first, InputIt last );		(5)
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.

3-4) inserts value in the position as close as possible, just prior(since C++11), to hint.

5) Inserts elements from range [first, last).

6) Inserts elements from initializer list ilist.

No iterators or references are invalidated.

Parameters

hint

iterator, used as a suggestion as to where to start the search	(until C++11)
iterator to the position before which the new element will be inserted	(since C++11)

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

1-2) Logarithmic in the size of the container, O(log(size())).

3-4) Amortized constant if the insertion happens in the position just after the hint, logarithmic in the size of the container otherwise.	(until C++11)
3-4) Amortized constant if the insertion happens in the position just before the hint, logarithmic in the size of the container otherwise.	(since C++11)

5-6) O(N*log(size() + N)), where N is the number of elements to insert.

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.

The overloads (5,6) are often implemented as a loop that calls the overload (3) with end() as the hint; they are optimized for appending a sorted sequence (such as another set) whose smallest element is greater than the last element in *this.

Example

#include <set>
#include <cassert>
#include <iostream>
 
int main()
{
  std::set<int> set;
 
  auto result_1 = set.insert(3);
  assert(result_1.first != set.end()); // it's a valid iterator
  assert(*result_1.first == 3);
  if (result_1.second)
    std::cout << "insert done\n";
 
  auto result_2 = set.insert(3);
  assert(result_2.first == result_1.first); // same iterator
  assert(*result_2.first == 3);
  if (!result_2.second)
    std::cout << "no insertion\n";
}

Output:

insert done
no insertion

emplace (C++11)	constructs element in-place (public member function)
emplace_hint (C++11)	constructs elements in-place using a hint (public member function)