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

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

	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)....

	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_set>

      const int n_operations = 100'500'0;

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

      std::size_t set_emplace_hint()
      {
	  std::unordered_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::unordered_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::unordered_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::unordered_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:
      146.88ms for plain emplace (ratio: 1.00)
      168.20ms for emplace with correct hint (ratio: 0.87)
      168.78ms for emplace with wrong hint (ratio: 0.87)
      166.58ms for corrected emplace (ratio: 0.88)
      168.27ms for emplace using returned iterator (ratio: 0.87)

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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