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std::unordered_map::operator[]

From cppreference.com
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.

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.

1) Inserts value_type(key, T())
-
key_type must meet the requirements of CopyConstructible.
-
mapped_type must meet the requirements of DefaultConstructible. (since C++11)
2) Inserts value_type(std::move(key), T())
-
key_type must meet the requirements of MoveConstructible. (since C++11)
-
mapped_type must meet the requirements of DefaultConstructible. (since C++11)

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 equal to or greater than max_load_factor()*bucket_count().

Contents

[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

Average case: constant, worst case: linear in size.


[edit] 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'

[edit] See also

access specified element with bounds checking
(public member function) [edit]
inserts an element or assigns to the current element if the key already exists
(public member function) [edit]