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

NAME
     std::map::emplace_hint - std::map::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  constructor   of	the   element	type   (value_type,   that   is,
     std::pair<const Key, T>)
	is  called  with exactly the same arguments as supplied to the function,
     forwarded
	with std::forward<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 <map>

      const int n_operations = 100'500'0;

      std::size_t map_emplace()
      {
	  std::map<int, char> map;
	  for (int i = 0; i < n_operations; ++i)
	      map.emplace(i, 'a');
	  return map.size();
      }

      std::size_t map_emplace_hint()
      {
	  std::map<int, char> map;
	  auto it = map.begin();
	  for (int i = 0; i < n_operations; ++i)
	  {
	      map.emplace_hint(it, i, 'b');
	      it = map.end();
	  }
	  return map.size();
      }

      std::size_t map_emplace_hint_wrong()
      {
	  std::map<int, char> map;
	  auto it = map.begin();
	  for (int i = n_operations; i > 0; --i)
	  {
	      map.emplace_hint(it, i, 'c');
	      it = map.end();
	  }
	  return map.size();
      }

      std::size_t map_emplace_hint_corrected()
      {
	  std::map<int, char> map;
	  auto it = map.begin();
	  for (int i = n_operations; i > 0; --i)
	  {
	      map.emplace_hint(it, i, 'd');
	      it = map.begin();
	  }
	  return map.size();
      }

      std::size_t map_emplace_hint_closest()
      {
	  std::map<int, char> map;
	  auto it = map.begin();
	  for (int i = 0; i < n_operations; ++i)
	      it = map.emplace_hint(it, i, 'e');
	  return map.size();
      }

      double time_it(std::function<std::size_t()> map_test,
		     std::string what = "", double ratio = 0.0)
      {
	  const auto start = std::chrono::system_clock::now();
	  const std::size_t map_size = map_test();
	  const auto stop = std::chrono::system_clock::now();
	  std::chrono::duration<double, std::milli> time = stop - start;
	  if (what.size() && map_size)
	      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(map_emplace); // cache warmup
	  const auto x = time_it(map_emplace, "plain emplace");
	  time_it(map_emplace_hint, "emplace with correct hint", x);
	  time_it(map_emplace_hint_wrong, "emplace with wrong hint", x);
	  time_it(map_emplace_hint_corrected, "corrected emplace", x);
	  time_it(map_emplace_hint_closest, "emplace using  returned  iterator",
     x);
      }

Possible output:
      365.08ms for plain emplace (ratio: 1.00)
       96.54ms for emplace with correct hint (ratio: 3.78)
      409.40ms for emplace with wrong hint (ratio: 0.89)
       85.57ms for corrected emplace (ratio: 4.27)
       84.31ms for emplace using returned iterator (ratio: 4.33)

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::map::emplace_hint(3)
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