NAME¶

std::map::operator[] - std::map::operator[]

Synopsis¶

T& operator[]( const Key& key ); (1)
T& operator[]( Key&& key ); (2) (since C++11)
template< class K > (3) (since C++26)
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 value_type(key, T()) if the key does not exist.

-
key_type must meet the requirements of CopyConstructible.
-
mapped_type must meet the requirements of CopyConstructible and (until C++11)
DefaultConstructible.

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 a value_type object constructed in-place from
std::piecewise_construct, std::forward_as_tuple(key), std::tuple<>()
if the key does not exist.
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.
(since C++11)
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.
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.

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.

No iterators or references are invalidated.

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.

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 not (since C++17)
require default-constructibility of the mapped type.

Feature-test macro Value Std Feature
Heterogeneous
overloads for the
remaining member
__cpp_lib_associative_heterogeneous_insertion 202311L (C++26) functions in ordered
and unordered
associative
containers. (3)

Example¶

// Run this code

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the effect of overload (1) was simply
LWG 334 C++98 returning provided its own
(*((insert(std::make_pair(x, description instead
T()))).first)).second

See also¶

at access specified element with bounds checking
(public member function)
insert_or_assign inserts an element or assigns to the current element if the key
(C++17) already exists
(public member function)
try_emplace inserts in-place if the key does not exist, does nothing if the key
(C++17) exists
(public member function)