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.
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 . |
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.
Example
This example demonstrates how to modify existing values and insert new values using operator[]
:
#include <iostream> #include <unordered_map> int main() { std::unordered_map<char, int> letter_counts {{'a', 27}, {'b', 3}, {'c', 1}}; std::cout << "initially:\n"; for (const auto &pair : letter_counts) { std::cout << pair.first << ": " << pair.second << '\n'; } letter_counts['b'] = 42; // update an existing value letter_counts['x'] = 9; // insert a new value std::cout << "after modifications:\n"; for (const auto &pair : letter_counts) { std::cout << pair.first << ": " << pair.second << '\n'; } }
Output:
initially: a: 27 b: 3 c: 1 after modifications: a: 27 b: 42 c: 1 x: 9
The following example counts the occurrences of each word in a vector of strings:
#include <string> #include <iostream> #include <vector> #include <unordered_map> int main() { std::vector<std::string> words = { "this", "sentence", "is", "not", "a", "sentence", "this", "sentence", "is", "a", "hoax" }; std::unordered_map<std::string, size_t> word_map; for (const auto &w : words) { ++word_map[w]; } for (const auto &pair : word_map) { std::cout << pair.second << " occurrences of word '" << pair.first << "'\n"; } }
Output:
1 occurrences of word 'hoax' 2 occurrences of word 'this' 2 occurrences of word 'a' 2 occurrences of word 'is' 1 occurrences of word 'not' 3 occurrences of word 'sentence'
See also
access specified element with bounds checking (public member function) | |
(C++17) | inserts an element or assigns to the current element if the key already exists (public member function) |
(C++17) | inserts in-place if the key does not exist, does nothing if the key exists (public member function) |
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