std::unordered_map::operator[](3)

NAME
std::unordered_map::operator[] - std::unordered_map::operator[]

Synopsis
   T& operator[]( const Key& key ); (1) (since C++11)
   T& operator[]( Key&& key ); (2) (since C++11)
   template< class K > (3) (since C++26)
   T& operator[]( K&& x );
   Returns a reference to the value that is mapped to a key equivalent to key or x
   respectively, performing an insertion if such key does not already exist.
   1) Inserts a value_type object constructed in-place from std::piecewise_construct,
   std::forward_as_tuple(key), std::tuple<>() if the key does not exist.
   Equivalent to return this->try_emplace(key).first->second;.
   (since C++17)When the default allocator is used, this results in the key being copy
   constructed from key and the mapped value being value-initialized.
   -
   value_type must be EmplaceConstructible from std::piecewise_construct,
   std::forward_as_tuple(key), std::tuple<>(). When the default allocator is used, this
   means that key_type must be CopyConstructible and mapped_type must be
   DefaultConstructible.
   2) Inserts a value_type object constructed in-place from std::piecewise_construct,
   std::forward_as_tuple(std::move(key)), std::tuple<>() if the key does not exist.
   Equivalent to return this->try_emplace(std::move(key)).first->second;.
   (since C++17)
   When the default allocator is used, this results in the key being move constructed
   from key and the mapped value being value-initialized.
   -
   value_type must be EmplaceConstructible from std::piecewise_construct,
   std::forward_as_tuple(std::move(key)), std::tuple<>(). When the default allocator is
   used, this means that key_type must be MoveConstructible and mapped_type must be
   DefaultConstructible.
   3) Inserts a value_type object constructed in-place if there is no key that
   transparently compares equivalent to the value x.
   Equivalent to return this->try_emplace(std::forward<K>(x)).first->second;. This
   overload participates in overload resolution only if Hash::is_transparent and
   KeyEqual::is_transparent are valid and each denotes a type. This assumes that such
   Hash is callable with both K and Key type, and that the KeyEqual is transparent,
   which, together, allows calling this function without constructing an instance of
   Key.
   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
   key - the key of the element to find
   x - a value of any type that can be transparently compared with a key

Return value
   1,2) A reference to the mapped value of the new element if no element with key key
   existed. Otherwise, a reference to the mapped value of the existing element whose
   key is equivalent to key.
   3) A reference to the mapped value of the new element if no element with key that
   compares equivalent to the value x existed. Otherwise, a reference to the mapped
   value of the existing element whose key compares equivalent to x.

Exceptions
   If an exception is thrown by any operation, the insertion has no effect.

Complexity
   Average case: constant, worst case: linear in size.

Notes
   In the published C++11 and C++14 standards, this function was specified to require
   mapped_type to be DefaultInsertable and key_type to be CopyInsertable or
   MoveInsertable into *this. This specification was defective and was fixed by LWG
   issue 2469, and the description above incorporates the resolution of that issue.
   However, one implementation (libc++) is known to construct the key_type and
   mapped_type objects via two separate allocator construct() calls, as arguably
   required by the standards as published, rather than emplacing a value_type object.
   operator[] is non-const because it inserts the key if it doesn't exist. If this
   behavior is undesirable or if the container is const, at may be used.
   insert_or_assign returns more information than operator[] and does not (since C++17)
   require default-constructibility of the mapped type.
                Feature-test macro Value Std Feature
                        Heterogeneous
                        overloads for the
                        remaining member
   __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) functions in ordered
                        and unordered
                        associative
                        containers. (3)

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <unordered_map>
 void println(auto const comment, auto const& map)
 {
     std::cout << comment << '{';
     for (const auto& pair : map)
         std::cout << '{' << pair.first << ": " << pair.second << '}';
     std::cout << "}\n";
 }
 int main()
 {
     std::unordered_map<char, int> letter_counts{{'a', 27}, {'b', 3}, {'c', 1}};
     println("letter_counts initially contains: ", letter_counts);
     letter_counts['b'] = 42; // updates an existing value
     letter_counts['x'] = 9; // inserts a new value
     println("after modifications it contains: ", letter_counts);
     // count the number of occurrences of each word
     // (the first call to operator[] initialized the counter with zero)
     std::unordered_map<std::string, int> word_map;
     for (const auto& w : {"this", "sentence", "is", "not", "a", "sentence",
                        "this", "sentence", "is", "a", "hoax"})
         ++word_map[w];
     word_map["that"]; // just inserts the pair {"that", 0}
     for (const auto& [word, count] : word_map)
         std::cout << count << " occurrence(s) of word '" << word << "'\n";
 }

Possible output:
 letter_counts initially contains: {{a: 27}{b: 3}{c: 1}}
 after modifications it contains: {{a: 27}{b: 42}{c: 1}{x: 9}}
 2 occurrence(s) of word 'a'
 1 occurrence(s) of word 'hoax'
 2 occurrence(s) of word 'is'
 1 occurrence(s) of word 'not'
 3 occurrence(s) of word 'sentence'
 0 occurrence(s) of word 'that'
 2 occurrence(s) of word 'this'

See also
   at access specified element with bounds checking
                    (public member function)
   insert_or_assign inserts an element or assigns to the current element if the key
   (C++17) already exists
                    (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)