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std::set<Key,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 element is constructed in-place, i.e. no copy or move operations are performed.

The constructor of the element 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

Returns an iterator to the newly inserted element.

If the insertion failed because the element already exists, returns an iterator to the already existing element with the equivalent key.

Exceptions

If an exception is thrown by any operation, this function has no effect (strong exception 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 <functional>
#include <iomanip>
#include <iostream>
#include <set>
 
const int n_operations = 100500;
 
std::size_t set_emplace() {
    std::set<int> set;
    for(int i = 0; i < n_operations; ++i) {
        set.emplace(i);
    }
    return set.size();
}
 
std::size_t set_emplace_hint() {
    std::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::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::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::set<int> set;
    auto it = set.begin();
    for(int i = 0; i < n_operations; ++i) {
        it = set.emplace_hint(it, i);
    }
    return set.size();
}
 
void timeit(std::function<std::size_t()> set_test, const char* what = nullptr) {
    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(6) << time.count() << "  ms for " << what << '\n';
    }
}
 
int main() {
    std::cout << std::fixed << std::setprecision(2);
    timeit(set_emplace); // stack warmup
    timeit(set_emplace, "plain emplace");
    timeit(set_emplace_hint, "emplace with correct hint");
    timeit(set_emplace_hint_wrong, "emplace with wrong hint");
    timeit(set_emplace_hint_corrected, "corrected emplace");
    timeit(set_emplace_hint_closest, "emplace using returned iterator");
}

Possible output:

 25.50  ms for plain emplace
  9.79  ms for emplace with correct hint
 28.49  ms for emplace with wrong hint
  8.01  ms for corrected emplace
  8.13  ms for emplace using returned iterator