W3cubDocs

/C++

std::map<Key,T,Compare,Allocator>::emplace_hint

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

(since C++11)

Inserts a new element to 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 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)....

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 <map>
 
const int n_operations = 100500;
 
std::size_t map_emplace() {
    std::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::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::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::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::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();
}
 
void timeit(std::function<std::size_t()> map_test, std::string what = "") {
    auto start = std::chrono::system_clock::now();
    std::size_t mapsize = map_test();
    auto stop = std::chrono::system_clock::now();
    std::chrono::duration<double, std::milli> time = stop - start;
    if (what.size() > 0 && mapsize > 0) {
        std::cout << std::setw(5) << time.count() << "  ms for " << what << '\n';
    }
}
 
int main() {
    std::cout << std::fixed << std::setprecision(2);
    timeit(map_emplace); // stack warmup
    timeit(map_emplace, "plain emplace");
    timeit(map_emplace_hint, "emplace with correct hint");
    timeit(map_emplace_hint_wrong, "emplace with wrong hint");
    timeit(map_emplace_hint_corrected, "corrected emplace");
    timeit(map_emplace_hint_closest, "emplace using returned iterator");
}

Possible output:

22.64  ms for plain emplace
 8.81  ms for emplace with correct hint
22.27  ms for emplace with wrong hint
 7.76  ms for corrected emplace
 8.30  ms for emplace using returned iterator