std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::try_emplace
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Tables
std::flat_map
flat_map::try_emplace
template< class... Args >
std::pair <iterator, bool> try_emplace( const key_type& k, Args&&... args );
(1)
(since C++23)
std::pair <iterator, bool> try_emplace( const key_type& k, Args&&... args );
template< class... Args >
std::pair <iterator, bool> try_emplace( key_type&& k, Args&&... args );
(2)
(since C++23)
std::pair <iterator, bool> try_emplace( key_type&& k, Args&&... args );
template< class K, class... Args >
std::pair <iterator, bool> try_emplace( K&& k, Args&&... args );
(3)
(since C++23)
std::pair <iterator, bool> try_emplace( K&& k, Args&&... args );
template< class... Args >
iterator try_emplace( const_iterator hint, const key_type& k, Args&&... args );
(4)
(since C++23)
iterator try_emplace( const_iterator hint, const key_type& k, Args&&... args );
template< class... Args >
iterator try_emplace( const_iterator hint, key_type&& k, Args&&... args );
(5)
(since C++23)
iterator try_emplace( const_iterator hint, key_type&& k, Args&&... args );
template< class K, class... Args >
iterator try_emplace( const_iterator hint, K&& k, Args&&... args );
(6)
(since C++23)
iterator try_emplace( const_iterator hint, K&& k, Args&&... args );
If a key equivalent to k already exists in the container, does nothing. Otherwise, inserts a new element into the underlying containers c with key k and value constructed with args.
1,2,4,5) Equivalent to:
auto key_it = ranges::upper_bound (c.keys, k, compare); auto value_it = c.values.begin() + std::distance (c.keys.begin(), key_it); c.keys.insert(key_it, std::forward <decltype(k)>(k)); c.values.emplace(value_it, std::forward <Args>(args)...);
3,6) Equivalent to:
auto key_it = ranges::upper_bound (c.keys, k, compare); auto value_it = c.values.begin() + std::distance (c.keys.begin(), key_it); c.keys.emplace(key_it, std::forward <K>(k)); c.values.emplace(value_it, std::forward <Args>(args)...);
The conversion from k into
key_type must construct an object u, for which find(k) == find(u) is true. Otherwise, the behavior is undefined. These overloads participate in overload resolution only if :
- The qualified-id
Compare::is_transparentis valid and denotes a type. - std::is_constructible_v <key_type, K> is true.
- std::is_assignable_v <mapped_type&, Args...> is true.
- For (3) only, std::is_convertible_v <K&&, const_iterator> and std::is_convertible_v <K&&, iterator> are both false.
Information on iterator invalidation is copied from here
[edit] Parameters
k
-
the key used both to look up and to insert if not found
hint
-
iterator to the position before which the new element will be inserted
args
-
arguments to forward to the constructor of the element
[edit] Return value
1-3) Same as for
emplace.4-6) Same as for
emplace_hint.[edit] Complexity
1-3) Same as for
emplace.4-6) Same as for
emplace_hint.[edit] 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::flat_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 ).
Overloads (3,6) can be called without constructing an object of type key_type.
[edit] Example
Run this code
#include <flat_map> #include <iostream> #include <string> #include <utility> void print_node(const auto& node) { std::cout << '[' << node.first << "] = " << node.second << '\n'; } void print_result(auto const& pair) { std::cout << (pair.second ? "inserted: " : "ignored: "); print_node(*pair.first); } int main() { using namespace std::literals; std::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); }
Output:
inserted: [a] = a inserted: [b] = abcd inserted: [c] = cccccccccc ignored: [c] = cccccccccc [a] = a [b] = abcd [c] = cccccccccc
[edit] See also
inserts an element or assigns to the current element if the key already exists
(public member function) [edit]
(public member function) [edit]