std::set::emplace_hint

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

Inserts a new element into the container as close as possible to the position just before hint. The element is constructed in-place, i.e. no copy or move operations are performed.

The constructor of the element 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 inserted
args	-	arguments to forward to the constructor of the element

Return value

Returns an iterator to the newly inserted element.

If the insertion failed because the element already exists, returns an iterator to the already existing element with the equivalent key.

Exceptions

If an exception is thrown by any operation, this function has no effect (strong exception guarantee).

Complexity

Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint.

Example

Run this code

#include <set>
#include <chrono>
#include <iostream>
#include <iomanip>
#include <functional>
 
const int nof_operations = 100500;
 
int set_emplace() {
  std::set<int> set;
  for(int i = 0; i < nof_operations; ++i) {
    set.emplace(i);
  }
  return set.size();
}
 
int set_emplace_hint() {
  std::set<int> set;
  auto it = set.begin();
  for(int i = 0; i < nof_operations; ++i) {
    set.emplace_hint(it, i);
    it = set.end();
  }
  return set.size();
}
 
int set_emplace_hint_wrong() {
  std::set<int> set;
  auto it = set.begin();
  for(int i = nof_operations; i > 0; --i) {
    set.emplace_hint(it, i);
    it = set.end();
  }
  return set.size();
}
 
int set_emplace_hint_corrected() {
  std::set<int> set;
  auto it = set.begin();
  for(int i = nof_operations; i > 0; --i) {
    set.emplace_hint(it, i);
    it = set.begin();
  }
  return set.size();
}
 
int set_emplace_hint_closest() {
  std::set<int> set;
  auto it = set.begin();
  for(int i = 0; i < nof_operations; ++i) {
    it = set.emplace_hint(it, i);
  }
  return set.size();
}
 
void timeit(std::function<int()> set_test, std::string what = "") {
  auto start = std::chrono::system_clock::now();
  int setsize = set_test();
  auto stop = std::chrono::system_clock::now();
  std::chrono::duration<double, std::milli> time = stop - start;
  if (what.size() > 0 && setsize > 0) {
    std::cout << std::fixed << std::setprecision(2)
              << time.count() << "  ms for " << what << '\n';
  }
}
 
int main() {
   timeit(set_emplace); // stack warmup
   timeit(set_emplace, "plain emplace");
   timeit(set_emplace_hint, "emplace with correct hint");
   timeit(set_emplace_hint_wrong, "emplace with wrong hint");
   timeit(set_emplace_hint_corrected, "corrected emplace");
   timeit(set_emplace_hint_closest, "emplace using returned iterator");
}

Possible output:

18.96  ms for plain emplace
7.95  ms for emplace with correct hint
19.39  ms for emplace with wrong hint
8.39  ms for corrected emplace
7.90  ms for emplace using returned iterator

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

array (C++11)
vector
deque
forward_list (C++11)
list
set
multiset
map
multimap
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
stack
queue
priority_queue

Member functions
set::set
set::~set
set::operator=
set::get_allocator
Iterators
set::beginset::cbegin (C++11)
set::endset::cend (C++11)
set::rbeginset::crbegin (C++11)
set::rendset::crend (C++11)
Capacity
set::empty
set::size
set::max_size
Modifiers
set::clear
set::insert
set::emplace (C++11)
set::emplace_hint (C++11)
set::erase
set::swap
set::extract (C++17)
set::merge (C++17)
Lookup
set::count
set::find
set::equal_range
set::lower_bound
set::upper_bound
Observers
set::key_comp
set::value_comp