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

T& operator[]( const Key& key );

T& operator[]( Key&& key );

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 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

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.

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.

Example

#include <iostream>
#include <string>
#include <unordered_map>
 
auto print = [](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}};
 
    print("letter_counts initially contains: ", letter_counts);
 
    letter_counts['b'] = 42; // updates an existing value
    letter_counts['x'] = 9;  // inserts a new value
 
    print("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";
}

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