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std::map::emplace_hint(3) | C++ Standard Libary | std::map::emplace_hint(3) |
NAME¶
std::map::emplace_hint - std::map::emplace_hint
Synopsis¶
template <class... Args> (since C++11)
iterator emplace_hint( const_iterator hint, Args&&... args );
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 constructs element in-place
(C++11) (public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
2022.07.31 | http://cppreference.com |