NAME

std::flat_set::emplace_hint - std::flat_set::emplace_hint

Synopsis

template< class... Args > (since C++23)
iterator emplace_hint( const_iterator hint, Args&&... args );

Inserts a new element into the container as close as possible to the position just
before hint.

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)....

Information on iterator invalidation is copied from here

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

This section is incomplete

Example

// Run this code

#include <chrono>
#include <cstddef>
#include <functional>
#include <iomanip>
#include <iostream>
#include <flat_set>

const int n_operations = 100'500'0;

std::size_t set_emplace()
{
std::flat_set<int> set;
for (int i = 0; i < n_operations; ++i)
set.emplace(i);
return set.size();
}

std::size_t set_emplace_hint()
{
std::flat_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::flat_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::flat_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::flat_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:

...TODO...

See also

emplace constructs element in-place
(public member function)
insert inserts elements
(public member function)

Category:

* Todo without reason