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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

Views

span (C++20)

 

std::map

Member functions

map::map

map::~map

map::operator=

map::get_allocator

Element access

map::at

map::operator[]

Iterators

map::beginmap::cbegin (C++11)

map::endmap::cend (C++11)

map::rbeginmap::crbegin (C++11)

map::rendmap::crend (C++11)

Capacity

map::empty

map::size

map::max_size

Modifiers

map::clear

map::insert

map::insert_or_assign (C++17)

map::emplace (C++11)

map::emplace_hint (C++11)

map::try_emplace (C++17)

map::erase

map::swap

map::extract (C++17)

map::merge (C++17)

Lookup

map::count

map::find

map::contains (C++20)

map::equal_range

map::lower_bound

map::upper_bound

Observers

map::key_comp

map::value_comp

Non-member functions

std::swap

erase_if (C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

Deduction guides (C++17)

 

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

Run this code

#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

See also

emplace (C++11)	constructs element in-place (public member function)
insert	inserts elements or nodes (since C++17) (public member function)