std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::operator[]

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 and KeyEqual are both transparent. 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.

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 require default-constructibility of the mapped type.

(since C++17)

Feature-test macro	Value	Std	Feature
`__cpp_lib_associative_heterogeneous_insertion`	`202311L`	(C++26)	Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. (3)

[edit] 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'