NAME¶

std::unordered_set::emplace_hint - std::unordered_set::emplace_hint

Synopsis¶

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

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.

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_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)

See also¶

emplace constructs element in-place
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)