std::unordered_map try_emplace() method
- od C++17
// (1) Non const version only
template< class... Args >
pair<iterator, bool> try_emplace( const Key& k, Args&&... args );
// (2) Non const version only
template< class... Args >
pair<iterator, bool> try_emplace( Key&& k, Args&&... args );
// (3) Non const version only
template< class... Args >
iterator try_emplace( const_iterator hint, const Key& k, Args&&... args );
// (4) Non const version only
template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&... args );
Inserts a new element into the container with key k
and value constructed with args
, if there is no element with the key in the container.
-
(1) If a key equivalent to
k
already exists in the container, does nothing. Otherwise, behaves likeemplace()
except that the element is constructed asvalue_type(std::piecewise_construct,
std::forward_as_tuple(k),
std::forward_as_tuple(std::forward<Args>(args)...)) -
(2) If a key equivalent to
k
already exists in the container, does nothing. Otherwise, behaves likeemplace()
except that the element is constructed asvalue_type(std::piecewise_construct,
std::forward_as_tuple(std::move(k)),
std::forward_as_tuple(std::forward<Args>(args)...)) -
(3) If a key equivalent to
k
already exists in the container, does nothing. Otherwise, behaves likeemplace_hint()
except that the element is constructed asvalue_type(std::piecewise_construct,
std::forward_as_tuple(k),
std::forward_as_tuple(std::forward<Args>(args)...)) -
(4) If a key equivalent to
k
already exists in the container, does nothing. Otherwise, behaves likeemplace_hint()
except that the element is constructed asvalue_type(std::piecewise_construct,
std::forward_as_tuple(std::move(k)),
std::forward_as_tuple(std::forward<Args>(args)...))
If rehashing occurs due to the insertion, all iterators are invalidated.
Otherwise iterators are not affected. References are not invalidated.
Rehashing occurs only if the new number of elements is greater than max_load_factor() * bucket_count()
.
Parameters
k
- the key used both to look up and to insert if not foundhint
- iterator to the position before which the new element will be insertedargs
- arguments to forward to the constructor of the element
Return value
- (1-2) Same as for
emplace()
. - (3-4) Same as for
emplace_hint()
.
Complexity
- (1-2) Same as for
emplace()
. - (3-4) Same as for
emplace_hint()
.
Exceptions
If an exception is thrown by any operation, this function has no effect (strong exception guarantee).
Notes
Unlike insert()
or emplace()
,
these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types,
such as std::unordered_map<std::string, std::unique_ptr<foo>>
.
In addition, try_emplace()
treats the key and the arguments to the mapped_type
separately, unlike emplace, which requires the arguments to construct a value_type
(that is, a std::pair
).
Feature testing macro: __cpp_lib_unordered_map_try_emplace
.
Example
#include <iostream>
#include <utility>
#include <string>
#include <unordered_map>
auto print_node = [](const auto &node) {
std::cout << "[" << node.first << "] = " << node.second << '\n';
};
auto print_result = [](auto const &pair) {
std::cout << (pair.second ? "inserted: " : "ignored: ");
print_node(*pair.first);
};
int main()
{
using namespace std::literals;
std::unordered_map<std::string, std::string> m;
print_result( m.try_emplace("a", "a"s) );
print_result( m.try_emplace("b", "abcd") );
print_result( m.try_emplace("c", 10, 'c') );
print_result( m.try_emplace("c", "Won't be inserted") );
for (const auto &p : m) { print_node(p); }
}
inserted: [a] = a
inserted: [b] = abcd
inserted: [c] = cccccccccc
ignored: [c] = cccccccccc
[a] = a
[b] = abcd
[c] = cccccccccc
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