table of contents
std::map::emplace(3) | C++ Standard Libary | std::map::emplace(3) |
NAME¶
std::map::emplace - std::map::emplace
Synopsis¶
template< class... Args > (since C++11)
std::pair<iterator,bool> emplace( Args&&... args );
Inserts a new element into the container constructed in-place with the given
args if
there is no element with the key in the container.
Careful use of emplace allows the new element to be constructed while
avoiding
unnecessary copy or move operations. The constructor of the new element (i.e.
std::pair<const Key, T>) is called with exactly the same arguments as
supplied to
emplace, forwarded via std::forward<Args>(args).... The element may be
constructed
even if there already is an element with the key in the container, in which
case the
newly constructed element will be destroyed immediately.
No iterators or references are invalidated.
Parameters¶
args - arguments to forward to the constructor of the element
Return value¶
Returns a pair consisting of an iterator to the inserted element,
or the
already-existing element if no insertion happened, and a bool denoting
whether the
insertion took place (true if insertion happened, false if it did not).
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.
Example¶
// Run this code
#include <iostream>
#include <utility>
#include <string>
#include <map>
int main()
{
std::map<std::string, std::string> m;
// uses pair's move constructor
m.emplace(std::make_pair(std::string("a"),
std::string("a")));
// uses pair's converting move constructor
m.emplace(std::make_pair("b", "abcd"));
// uses pair's template constructor
m.emplace("d", "ddd");
// uses pair's piecewise constructor
m.emplace(std::piecewise_construct,
std::forward_as_tuple("c"),
std::forward_as_tuple(10, 'c'));
// as of C++17, m.try_emplace("c", 10, 'c'); can be used
for (const auto &p : m) {
std::cout << p.first << " => " << p.second
<< '\n';
}
}
Output:¶
a => a
b => abcd
c => cccccccccc
d => ddd
See also¶
emplace_hint constructs elements in-place using a hint
(C++11) (public member function)
try_emplace inserts in-place if the key does not exist, does nothing if the
key
(C++17) exists
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
2022.07.31 | http://cppreference.com |