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)
   Returns a reference to the value that is mapped to a key equivalent to key,
   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.
   This function is 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.
   This function is 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.
   If an insertion occurs and results in a rehashing of the container, 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
   key - the key of the element to find

Return value
   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.

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 (since C++17)
   not require default-constructibility of the mapped type.

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <unordered_map>
 auto print = [](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}};
     print("letter_counts initially contains: ", letter_counts);
     letter_counts['b'] = 42; // updates an existing value
     letter_counts['x'] = 9; // inserts a new value
     print("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 << " occurrences 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 occurrences of word 'a'
 1 occurrences of word 'hoax'
 2 occurrences of word 'is'
 1 occurrences of word 'not'
 3 occurrences of word 'sentence'
 0 occurrences of word 'that'
 2 occurrences of word 'this'

See also
   at access specified element with bounds checking
   (C++11) (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)