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std::unorde...mplace_hint(3)   C++ Standard Libary  std::unorde...mplace_hint(3)

NAME
     std::unordered_map::emplace_hint - std::unordered_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, using  hint  as  a  suggestion
     where the
	element should go.

	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)....

	If after the operation the new number of elements is greater than old
	max_load_factor() * bucket_count() a rehashing takes place.
	If  rehashing  occurs  (due to the insertion), all iterators are invali-
     dated. Otherwise
	(no rehashing), iterators are not invalidated.

Parameters
	hint - iterator, used as a suggestion as to where to insert the new ele-
     ment
	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
	Amortized constant on average, worst case linear in the size of the con-
     tainer.

Example
     // Run this code

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

      const int n_operations = 100'500'0;

      std::size_t map_emplace()
      {
	  std::unordered_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::unordered_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::unordered_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::unordered_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::unordered_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:
      143.48ms for plain emplace (ratio: 1.00)
      164.78ms for emplace with correct hint (ratio: 0.87)
      171.22ms for emplace with wrong hint (ratio: 0.84)
      166.55ms for corrected emplace (ratio: 0.86)
      167.41ms for emplace using returned iterator (ratio: 0.86)

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

http://cppreference.com 	   2024.06.10	    std::unorde...mplace_hint(3)
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