std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace_hint

 

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std::unordered_map

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unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
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operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
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template< class... Args > iterator emplace_hint( const_iterator hint, Args&&... args );		(since C++11)

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 invalidated. Otherwise (no rehashing), iterators are not invalidated.

Parameters

hint	-	iterator, used as a suggestion as to where to insert the new element
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 container.

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)
insert	inserts elements or nodes(since C++17) (public member function)