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

NAME
std::flat_map::operator[] - std::flat_map::operator[]

Synopsis
   T& operator[]( const Key& key ); (1) (since C++23)
   T& operator[]( Key&& key ); (2) (since C++23)
   template< class K > (3) (since C++23)
   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 if the key does not exist.
   Equivalent to return try_emplace(x).first->second;.
   2) Inserts a value_type object constructed in-place if the key does not exist.
   Equivalent to return try_emplace(std::move(x)).first->second;
   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 the qualified-id
   Compare::is_transparent is valid and denotes a type. It allows calling this function
   without constructing an instance of Key.
    Information on iterator invalidation is copied from here

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
   Logarithmic in the size of the container, plus the cost of insertion (if any) of an
   empty element.

Notes
   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 require
   default-constructibility of the mapped type.

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <flat_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::flat_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::flat_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";
 }

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)
                    inserts an element or assigns to the current element if the key
   insert_or_assign already exists
                    (public member function)
                    inserts in-place if the key does not exist, does nothing if the key
   try_emplace exists
                    (public member function)