std::map<Key,T,Compare,Allocator>::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 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 inserted
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 constant if the new element is inserted just before hint.

Example

#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)