std::map<Key,T,Compare,Allocator>::operator[]
| T& operator[]( const Key& key ); |
(1) | |
| 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.
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1) Inserts value_type(key, T()) if the key does not exist.
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(until C++11) | ||||||
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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.
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.
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(since C++11) | ||||||
value_type object constructed in-place if there is no key that transparently compares equivalent to the value x.Key.No iterators or references are 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
Exceptions
If an exception is thrown by any operation, the insertion has no effect.
Complexity
Logarithmic in the size of the container.
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.
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(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
#include <iostream> #include <string> #include <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::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::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'Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 334 | C++98 | the effect of overload (1) was simply returning (*((insert(std::make_pair(x, T()))).first)).second |
provided its own description instead |
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) |