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std::set::emplace_hint(3)      C++ Standard Libary     std::set::emplace_hint(3)

NAME
     std::set::emplace_hint - std::set::emplace_hint

Synopsis
	template<  class...  Args  >					  (since
     C++11)
	iterator emplace_hint( const_iterator hint, Args&&... args );

	Inserts a new element into the container as close as possible to the po-
     sition just
	before hint.

	The constructors of the key and mapped value are called with exactly the
     same
	arguments  as  supplied  to  the  function,  forwarded	with   std::for-
     ward<Args>(args)....

	No iterators or references are invalidated.

Parameters
	hint - iterator to the position before which the new element will be in-
     serted
	args - arguments to forward to the constructor of the element

Return value
	An  iterator  to  the inserted element, or to the element that prevented
     the insertion.

Exceptions
	If an exception is thrown for any reason, this function  has  no  effect
     (strong
	exception safety guarantee).

Complexity
	Logarithmic  in the size of the container in general, but amortized con-
     stant if the
	new element is inserted just before hint.

Example
     // Run this code

      #include <chrono>
      #include <cstddef>
      #include <functional>
      #include <iomanip>
      #include <iostream>
      #include <set>

      const int n_operations = 100'500'0;

      std::size_t set_emplace()
      {
	  std::set<int> set;
	  for (int i = 0; i < n_operations; ++i)
	      set.emplace(i);
	  return set.size();
      }

      std::size_t set_emplace_hint()
      {
	  std::set<int> set;
	  auto it = set.begin();
	  for (int i = 0; i < n_operations; ++i)
	  {
	      set.emplace_hint(it, i);
	      it = set.end();
	  }
	  return set.size();
      }

      std::size_t set_emplace_hint_wrong()
      {
	  std::set<int> set;
	  auto it = set.begin();
	  for (int i = n_operations; i > 0; --i)
	  {
	      set.emplace_hint(it, i);
	      it = set.end();
	  }
	  return set.size();
      }

      std::size_t set_emplace_hint_corrected()
      {
	  std::set<int> set;
	  auto it = set.begin();
	  for (int i = n_operations; i > 0; --i)
	  {
	      set.emplace_hint(it, i);
	      it = set.begin();
	  }
	  return set.size();
      }

      std::size_t set_emplace_hint_closest()
      {
	  std::set<int> set;
	  auto it = set.begin();
	  for (int i = 0; i < n_operations; ++i)
	      it = set.emplace_hint(it, i);
	  return set.size();
      }

      double time_it(std::function<std::size_t()> set_test,
		     const char* what = nullptr,
		     double ratio = 0.0)
      {
	  const auto start = std::chrono::system_clock::now();
	  const std::size_t setsize = set_test();
	  const auto stop = std::chrono::system_clock::now();
	  const std::chrono::duration<double, std::milli> time = stop - start;
	  if (what != nullptr && setsize > 0)
	      std::cout << std::setw(8) << time << " for " << what << "  (ratio:
     "
			<< (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n";
	  return time.count();
      }

      int main()
      {
	  std::cout << std::fixed << std::setprecision(2);
	  time_it(set_emplace); // cache warmup
	  const auto x = time_it(set_emplace, "plain emplace");
	  time_it(set_emplace_hint, "emplace with correct hint", x);
	  time_it(set_emplace_hint_wrong, "emplace with wrong hint", x);
	  time_it(set_emplace_hint_corrected, "corrected emplace", x);
	  time_it(set_emplace_hint_closest,  "emplace  using returned iterator",
     x);
      }

Possible output:
      392.25ms for plain emplace (ratio: 1.00)
       97.15ms for emplace with correct hint (ratio: 4.04)
      387.52ms for emplace with wrong hint (ratio: 1.01)
       84.80ms for corrected emplace (ratio: 4.63)
       83.67ms for emplace using returned iterator (ratio: 4.69)

See also
	emplace constructs element in-place
	(C++11) (public member function)
		inserts elements
	insert	or nodes
		(since C++17)
		(public member function)

http://cppreference.com 	   2024.06.10	       std::set::emplace_hint(3)
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