std::map::operator[]
T& operator[]( const Key& key );
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(1) | |
T& operator[]( Key&& key );
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(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.
If an insertion is performed, the mapped value is value-initialized (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned.
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key_type must meet the requirements of CopyConstructible .
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mapped_type must meet the requirements of DefaultConstructible . (since C++11)
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key_type must meet the requirements of MoveConstructible . (since C++11)
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mapped_type must meet the requirements of DefaultConstructible . (since C++11)
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No iterators or references are invalidated.
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[edit] Parameters
key | - | the key of the element to find |
[edit] 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
.
[edit] Exceptions
If an exception is thrown by any operation, the insertion has no effect
[edit] Complexity
Logarithmic in the size of the container.
Notes
Until C++11, the overload (1) was specified to be equivalent to (insert(std::make_pair(key, T())).first)->second, which required T to be CopyConstructible.
[edit] Example
This example demonstrates how to modify existing values and insert new values using operator[]
:
#include <iostream> #include <map> int main() { std::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 <map> int main() { std::vector<std::string> words = { "this", "sentence", "is", "not", "a", "sentence", "this", "sentence", "is", "a", "hoax" }; std::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'
[edit] See also
(C++11)
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access specified element with bounds checking (public member function) |