std::map emplace_hint() method

since C++11

// Non const version only
template <class... Args>
iterator emplace_hint( const_iterator hint, Args&&... args );

Inserts a new element to 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 new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via 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.

Complexity

Logarithmic in the size of the container in general - O(log size()).
Amortized constant if the new element is inserted just before hint - O(1).

Exceptions

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

Example

Main.cpp

#include <chrono>
#include <iostream>
#include <iomanip>
#include <functional>
#include <map>
 
const int nof_operations = 100500;
 
int map_emplace() {
  std::map<int, char> map;
  for(int i = 0; i < nof_operations; ++i) {
    map.emplace(i, 'a');
  }
  return map.size();
}
 
int map_emplace_hint() {
  std::map<int, char> map;
  auto it = map.begin();
  for(int i = 0; i < nof_operations; ++i) {
    map.emplace_hint(it, i, 'b');
    it = map.end();
  }
  return map.size();
}
 
int map_emplace_hint_wrong() {
  std::map<int, char> map;
  auto it = map.begin();
  for(int i = nof_operations; i > 0; --i) {
    map.emplace_hint(it, i, 'c');
    it = map.end();
  }
  return map.size();
}
 
int map_emplace_hint_corrected() {
  std::map<int, char> map;
  auto it = map.begin();
  for(int i = nof_operations; i > 0; --i) {
    map.emplace_hint(it, i, 'd');
    it = map.begin();
  }
  return map.size();
}
 
int map_emplace_hint_closest() {
  std::map<int, char> map;
  auto it = map.begin();
  for(int i = 0; i < nof_operations; ++i) {
    it = map.emplace_hint(it, i, 'e');
  }
  return map.size();
}
 
void timeit(std::function<int()> map_test, std::string what = "") {
  auto start = std::chrono::system_clock::now();
  int mapsize = map_test();
  auto stop = std::chrono::system_clock::now();
  std::chrono::duration<double, std::milli> time = stop - start;
  if (what.size() > 0 && mapsize > 0) {
    std::cout << std::fixed << std::setprecision(2) << std::setw(5)
              << time.count() << "  ms for " << what << '\n';
  }
}
 
int main() {
   timeit(map_emplace); // stack warmup
   timeit(map_emplace, "plain emplace");
   timeit(map_emplace_hint, "emplace with correct hint");
   timeit(map_emplace_hint_wrong, "emplace with wrong hint");
   timeit(map_emplace_hint_corrected, "corrected emplace");
   timeit(map_emplace_hint_closest, "emplace using returned iterator");
}

Output

22.64  ms for plain emplace
 8.81  ms for emplace with correct hint
22.27  ms for emplace with wrong hint
 7.76  ms for corrected emplace
 8.30  ms for emplace using returned iterator