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Showing content from https://cplusplus.com/reference/set/set/insert/ below:

public member function

<set>

std::set::insert single element (1)

pair<iterator,bool> insert (const value_type& val);

with hint (2)

iterator insert (iterator position, const value_type& val);

range (3)

template <class InputIterator>  void insert (InputIterator first, InputIterator last);

single element (1)

pair<iterator,bool> insert (const value_type& val);pair<iterator,bool> insert (value_type&& val);

with hint (2)

iterator insert (const_iterator position, const value_type& val);iterator insert (const_iterator position, value_type&& val);

range (3)

template <class InputIterator>  void insert (InputIterator first, InputIterator last);

initializer list (4)

void insert (initializer_list<value_type> il);

Insert element

Extends the container by inserting new elements, effectively increasing the container size by the number of elements inserted.

Because elements in a set are unique, the insertion operation checks whether each inserted element is equivalent to an element already in the container, and if so, the element is not inserted, returning an iterator to this existing element (if the function returns a value).

For a similar container allowing for duplicate elements, see multiset.

Internally, set containers keep all their elements sorted following the criterion specified by its comparison object. The elements are always inserted in its respective position following this ordering.

The parameters determine how many elements are inserted and to which values they are initialized:

Parameters

val

Value to be copied (or moved) to the inserted elements.
Member type value_type is the type of the elements in the container, defined in set as an alias of its first template parameter (T).

position

Hint for the position where the element can be inserted.

The function optimizes its insertion time if position points to the element that will precede the inserted element.

The function optimizes its insertion time if

position

points to the element that will

follow

the inserted element (or to the

end

, if it would be the last).

Notice that this is just a hint and does not force the new element to be inserted at that position within the set container (the elements in a set always follow a specific order).
Member types iterator and const_iterator are defined in map as a bidirectional iterator type that point to elements.

first, last

Iterators specifying a range of elements. Copies of the elements in the range [first,last) are inserted in the container.
Notice that the range includes all the elements between first and last, including the element pointed by first but not the one pointed by last.
The function template argument InputIterator shall be an input iterator type that points to elements of a type from which value_type objects can be constructed.

il

An initializer_list object. Copies of these elements are inserted.
These objects are automatically constructed from initializer list declarators.
Member type value_type is the type of the elements in the container, defined in set as an alias of its first template parameter (T).

Return value The single element versions (1) return a pair, with its member pair::first set to an iterator pointing to either the newly inserted element or to the equivalent element already in the set. The pair::second element in the pair is set to true if a new element was inserted or false if an equivalent element already existed.

The versions with a hint (2) return an iterator pointing to either the newly inserted element or to the element that already had its same value in the set.

Member type iterator is a bidirectional iterator type that points to elements.

pair is a class template declared in <utility> (see pair).

Example

// set::insert (C++98)
#include <iostream>
#include <set>

int main ()
{
  std::set<int> myset;
  std::set<int>::iterator it;
  std::pair<std::set<int>::iterator,bool> ret;

  // set some initial values:
  for (int i=1; i<=5; ++i) myset.insert(i*10);    // set: 10 20 30 40 50

  ret = myset.insert(20);               // no new element inserted

  if (ret.second==false) it=ret.first;  // "it" now points to element 20

  myset.insert (it,25);                 // max efficiency inserting
  myset.insert (it,24);                 // max efficiency inserting
  myset.insert (it,26);                 // no max efficiency inserting

  int myints[]= {5,10,15};              // 10 already in set, not inserted
  myset.insert (myints,myints+3);

  std::cout << "myset contains:";
  for (it=myset.begin(); it!=myset.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Output:

myset contains: 5 10 15 20 24 25 26 30 40 50

Complexity If a single element is inserted, logarithmic in size in general, but amortized constant if a hint is given and the position given is the optimal.

If N elements are inserted,

Nlog(size+N)

in general, but linear in

size+N

if the elements are already sorted according to the same ordering criterion used by the container.

If N elements are inserted,

Nlog(size+N)

Implementations may optimize if the range is already sorted.

Iterator validity No changes.

Data races The container is modified.
Concurrently accessing existing elements is safe, although iterating ranges in the container is not.

Exception safety If a single element is to be inserted, there are no changes in the container in case of exception (strong guarantee).
Otherwise, the container is guaranteed to end in a valid state (basic guarantee).
If allocator_traits::construct is not supported with the appropriate arguments for the element constructions, or if an invalid position is specified, it causes undefined behavior.

See also

set::erase: Erase elements (public member function)

set::find: Get iterator to element (public member function)

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