std::unordered_map::try_emplace(3)

NAME
std::unordered_map::try_emplace - std::unordered_map::try_emplace

Synopsis
   template< class... Args >
   std::pair<iterator, bool> try_emplace( const Key& k, Args&&... (1) (since C++17)
   args );
   template< class... Args > (2) (since C++17)
   std::pair<iterator, bool> try_emplace( Key&& k, Args&&... args );
   template< class K, class... Args > (3) (since C++26)
   std::pair<iterator, bool> try_emplace( K&& k, Args&&... args );
   template< class... Args >
   iterator try_emplace( const_iterator hint, const Key& k, Args&&... (4) (since C++17)
   args );
   template< class... Args >
   iterator try_emplace( const_iterator hint, Key&& k, Args&&... args (5) (since C++17)
   );
   template< class K, class... Args >
   iterator try_emplace( const_iterator hint, K&& k, Args&&... args (6) (since C++26)
   );
   If a key equivalent to k already exists in the container, does nothing. Otherwise,
   inserts a new element into the container with key k and value constructed with args.
   In such case:
   1) Behaves like emplace except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(k),
              std::forward_as_tuple(std::forward<Args>(args)...))
   2) Behaves like emplace except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(std::move(k)),
              std::forward_as_tuple(std::forward<Args>(args)...))
   3) Behaves like emplace except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(std::forward<K>(k)),
              std::forward_as_tuple(std::forward<Args>(args)...))
   4) Behaves like emplace_hint except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(k),
              std::forward_as_tuple(std::forward<Args>(args)...))
   5) Behaves like emplace_hint except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(std::move(k)),
              std::forward_as_tuple(std::forward<Args>(args)...))
   6) Behaves like emplace_hint except that the element is constructed as
   value_type(std::piecewise_construct,
              std::forward_as_tuple(std::forward<K>(k)),
              std::forward_as_tuple(std::forward<Args>(args)...))
   1-6) If value_type is not EmplaceConstructible into unordered_map from the
   corresponding expression, the behavior is undefined.
   3) This overload participates in overload resolution only if all following
   conditions are satisfied:
     * std::is_convertible_v<K&&, const_iterator> and std::is_convertible_v<K&&,
       iterator> are both false.
     * Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a
       type.
   If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the
   behavior is undefined, where u is the new element to be inserted.
   6) This overload participates in overload resolution only if Hash::is_transparent
   and KeyEqual::is_transparent are both valid and each denotes a type.
   If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the
   behavior is undefined, where u is the new element to be inserted.
   If after the operation the new number of elements is greater than old
   max_load_factor() * bucket_count() a rehashing takes place.
   If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise
   (no rehashing), iterators are not invalidated.

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

Return value
   1-3) Same as for emplace.
   4-6) Same as for emplace_hint.

Complexity
   1-3) Same as for emplace.
   4-6) Same as for emplace_hint.

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).
   Overloads (3,6) can be called without constructing an object of type Key.
             Feature-test macro Value Std Feature __cpp_lib_unordered_map_try_emplace 201411L (C++17) std::unordered_map::try_emplace,
                        std::unordered_map::insert_or_assign
                        Heterogeneous overloads for the __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) remaining member functions in
                        ordered and unordered associative
                        containers. Overloads (3) and (6).

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <unordered_map>
 #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::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);
 }

Possible output:
 inserted: [a] = a
 inserted: [b] = abcd
 inserted: [c] = cccccccccc
 ignored: [c] = cccccccccc
 [a] = a
 [b] = abcd
 [c] = cccccccccc

See also
   emplace constructs element in-place
                (public member function)
   emplace_hint constructs elements in-place using a hint
                (public member function)
                inserts elements
   insert or nodes
                (since C++17)
                (public member function)