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

public member function

<set>

std::set::emplace_hint

template <class... Args>  iterator emplace_hint (const_iterator position, Args&&... args);

Construct and insert element with hint

Inserts a new element in the set, if unique, with a hint on the insertion position. This new element is constructed in place using args as the arguments for its construction.

The insertion only takes place if no other element in the container is equivalent to the one being emplaced (elements in a set container are unique).

If inserted, this effectively increases the container size by one.

The value in position is used as a hint on the insertion point. The element will nevertheless be inserted at its corresponding position following the order described by its internal comparison object, but this hint is used by the function to begin its search for the insertion point, speeding up the process considerably when the actual insertion point is either position or close to it.

The element is constructed in-place by calling allocator_traits::construct with args forwarded.

Parameters

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 follow the inserted element (or to the end, if it would be the last).
Notice that this 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).
const_iterator is a member type, defined as a bidirectional iterator type that points to elements.
args: Arguments forwarded to construct the new element.

Return value If the function successfully inserts the element (because no equivalent element existed already in the set), the function returns an iterator to the newly inserted element.

Otherwise, it returns an iterator to the equivalent element within the container.

Member type iterator is a bidirectional iterator type that points to an element.

Example

// set::emplace_hint
#include <iostream>
#include <set>
#include <string>

int main ()
{
  std::set<std::string> myset;
  auto it = myset.cbegin();

  myset.emplace_hint (it,"alpha");
  it = myset.emplace_hint (myset.cend(),"omega");
  it = myset.emplace_hint (it,"epsilon");
  it = myset.emplace_hint (it,"beta");

  std::cout << "myset contains:";
  for (const std::string& x: myset)
    std::cout << ' ' << x;
  std::cout << '\n';

  return 0;
}

Output:

myset contains: alpha beta epsilon omega

Complexity Generally, logarithmic in the container size.
Amortized constant if the insertion point for the element is position.

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 safetyStrong guarantee: if an exception is thrown, there are no changes in the container.
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::emplace: Construct and insert element (public member function)

set::insert: Insert element (public member function)

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

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