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.
Complexity
Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint.
Example
#include <set>
#include <chrono>
#include <iostream>
#include <iomanip>
#include <functional>
const int nof_operations = 100500;
int set_emplace() {
std::set<int> set;
for(int i = 0; i < nof_operations; ++i) {
set.emplace(i);
}
return set.size();
}
int set_emplace_hint() {
std::set<int> set;
auto it = set.begin();
for(int i = 0; i < nof_operations; ++i) {
set.emplace_hint(it, i);
it = set.end();
}
return set.size();
}
int set_emplace_hint_wrong() {
std::set<int> set;
auto it = set.begin();
for(int i = nof_operations; i > 0; --i) {
set.emplace_hint(it, i);
it = set.end();
}
return set.size();
}
int set_emplace_hint_corrected() {
std::set<int> set;
auto it = set.begin();
for(int i = nof_operations; i > 0; --i) {
set.emplace_hint(it, i);
it = set.begin();
}
return set.size();
}
int set_emplace_hint_closest() {
std::set<int> set;
auto it = set.begin();
for(int i = 0; i < nof_operations; ++i) {
it = set.emplace_hint(it, i);
}
return set.size();
}
void timeit(std::function<int()> set_test, std::string what = "") {
auto start = std::chrono::system_clock::now();
int setsize = set_test();
auto stop = std::chrono::system_clock::now();
std::chrono::duration<double, std::milli> time = stop - start;
if (what.size() > 0 && setsize > 0) {
std::cout << std::fixed << std::setprecision(2)
<< time.count() << " ms for " << what << '\n';
}
}
int main() {
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:
18.96 ms for plain emplace 7.95 ms for emplace with correct hint 19.39 ms for emplace with wrong hint 8.39 ms for corrected emplace 7.90 ms for emplace using returned iterator
See also
| (C++11) | constructs element in-place (public member function) |
| inserts elements (public member function) |
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