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

 

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std::unordered_map

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unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
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operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
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T& operator[]( const Key& key );	(1)	(since C++11)
T& operator[]( Key&& key );	(2)	(since C++11)
template< class K > T& operator[]( K&& x );	(3)	(since C++26)

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 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)

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 (C++17)	inserts an element or assigns to the current element if the key already exists (public member function)
try_emplace (C++17)	inserts in-place if the key does not exist, does nothing if the key exists (public member function)