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std::unordered_set<Key,Hash,KeyEqual,Allocator>::emplace_hint

From cppreference.com
 
 
 
 
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 constructors of the key and mapped value are 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.

Contents

[edit] 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

[edit] Return value

An iterator to the inserted element, or to the element that prevented the insertion.

[edit] Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

[edit] Complexity

Amortized constant on average, worst case linear in the size of the container.

[edit] Example

#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)

[edit] See also

constructs element in-place
(public member function) [edit]
inserts elements or nodes(since C++17)
(public member function) [edit]