std::map::operator[]

 

C++

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Utilities library

Strings library

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Iterators library

Numerics library

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Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

 

std::map

Member functions

map::map

map::~map

map::operator=

map::get_allocator

Element access

map::at

map::operator[]

Iterators

map::beginmap::cbegin (C++11)

map::endmap::cend (C++11)

map::rbeginmap::crbegin (C++11)

map::rendmap::crend (C++11)

Capacity

map::empty

map::size

map::max_size

Modifiers

map::clear

map::insert

map::insert_or_assign (C++17)

map::emplace (C++11)

map::emplace_hint (C++11)

map::try_emplace (C++17)

map::erase

map::swap

Lookup

map::count

map::find

map::equal_range

map::lower_bound

map::upper_bound

Observers

map::key_comp

map::value_comp

 

T& operator[]( const Key& key );	(1)
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 value_type(key, T()) if the key does not exist. This function is equivalent to return insert(std::make_pair(key, T())).first->second; - 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. 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.

(since C++11)

No iterators or references are invalidated.

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

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.

Example

This example demonstrates how to modify existing values and insert new values using operator[]:

Run this code

#include <iostream>
#include <map>
 
int main()
{
    std::map<char, int> letter_counts {{'a', 27}, {'b', 3}, {'c', 1}};
 
    std::cout << "initially:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
 
    letter_counts['b'] = 42;  // update an existing value
 
    letter_counts['x'] = 9;  // insert a new value
 
    std::cout << "after modifications:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
}

Output:

initially:
a: 27
b: 3
c: 1
after modifications:
a: 27
b: 42
c: 1
x: 9

The following example counts the occurrences of each word in a vector of strings:

Run this code

#include <string>
#include <iostream>
#include <vector>
#include <map>
 
int main()
{
    std::vector<std::string> words = { 
        "this", "sentence", "is", "not", "a", "sentence",
	"this", "sentence", "is", "a", "hoax" 
    };
 
    std::map<std::string, size_t>  word_map;
    for (const auto &w : words) {
        ++word_map[w];
    }
 
    for (const auto &pair : word_map) {
        std::cout << pair.second
	          << " occurrences of word '"
	          << pair.first << "'\n";
    }
}

Output:

1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'

See also

at (C++11)	access specified element with bounds checking (public member function)
insert_or_assign (C++17)	inserts an element or assigns to the current element if the key already exists (public member function)
try_emplace (C++17)	inserts in-place if the key does not exist, does nothing if the key exists (public member function)