NAME

std::unordered_map::operator[] - std::unordered_map::operator[]

Synopsis

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.

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.
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.

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.
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.

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.

insert_or_assign() returns more information than operator[] and does (since C++17)
not require default-constructibility of the mapped type.

Example

// Run this code

#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 << " occurrences of word '" << word << "'\n";
}
}

Possible output:

letter_counts initially contains: {{a: 27}{b: 3}{c: 1}}
after modifications it contains: {{a: 27}{b: 42}{c: 1}{x: 9}}
2 occurrences of word 'a'
1 occurrences of word 'hoax'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'
0 occurrences of word 'that'
2 occurrences of word 'this'