NAME¶

std::flat_map::operator[] - std::flat_map::operator[]

Synopsis¶

T& operator[]( const Key& key ); (1) (since C++23)
T& operator[]( Key&& key ); (2) (since C++23)
template< class K > (3) (since C++23)
T& operator[]( K&& x );

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.

1) Inserts a value_type object constructed in-place if the key does not exist.
Equivalent to return try_emplace(x).first->second;.
2) Inserts a value_type object constructed in-place if the key does not exist.
Equivalent to return try_emplace(std::move(x)).first->second;
3) Inserts a value_type object constructed in-place if there is no key that
transparently compares equivalent to the value x.
Equivalent to return this->try_emplace(std::forward<K>(x)).first->second;. This
overload participates in overload resolution only if the qualified-id
Compare::is_transparent is valid and denotes a type. It allows calling this function
without constructing an instance of Key.

Information on iterator invalidation is copied from here

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¶

1,2) A 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.
3) A reference to the mapped value of the new element if no element with key that
compares equivalent to the value x existed. Otherwise, a reference to the mapped
value of the existing element whose key compares equivalent to x.

Exceptions¶

If an exception is thrown by any operation, the insertion has no effect.

Complexity¶

Logarithmic in the size of the container, plus the cost of insertion (if any) of an
empty element.

Notes¶

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 not require
default-constructibility of the mapped type.

Example¶

// Run this code

#include <iostream>
#include <string>
#include <flat_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::flat_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::flat_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'

See also¶

at access specified element with bounds checking
(public member function)
inserts an element or assigns to the current element if the key
insert_or_assign already exists
(public member function)
inserts in-place if the key does not exist, does nothing if the key
try_emplace exists
(public member function)