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std::map<Key,T,Compare,Allocator>::operator[]

From cppreference.com
< cpp‎ | container‎ | map
 
 
 
 
T& operator[]( const Key& key );
(1)
T& operator[]( Key&& key );
(2) (since C++11)
template< class K >
T& operator[]( K&& x );
(3) (since C++26)

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 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.
(until C++11)
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.
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.
(since C++11)
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.

Contents

[edit] Parameters

key - the key of the element to find
x - a value of any type that can be transparently compared with a key

[edit] 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.

[edit] Exceptions

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

[edit] Complexity

Logarithmic in the size of the container.

[edit] 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 require default-constructibility of the mapped type.

(since C++17)
Feature-test macro Value Std Feature
__cpp_lib_associative_heterogeneous_insertion 202311L (C++26) Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. (3)

[edit] Example

#include <iostream>
#include <string>
#include <map>
 
void 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::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::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
LWG 334 C++98 the effect of overload (1) was simply returning
(*((insert(std::make_pair(x, T()))).first)).second
provided its own
description instead

[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]
inserts in-place if the key does not exist, does nothing if the key exists
(public member function) [edit]