std::map::operator[]

 

C++

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std::map

Member functions

map::map

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Element access

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map::operator[]

Iterators

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

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Capacity

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Observers

map::key_comp

map::value_comp

 

T& operator[]( const Key& key );	(1)
T& operator[]( Key&& key );	(2)	(since C++11)

Inserts a new element to the container using key as the key and a default constructed mapped value and returns a reference to the newly constructed mapped value. If an element with key key already exists, no insertion is performed and a reference to its mapped value is returned.

1) Essentially performs (insert(std::make_pair(key, T())).first)->second

2) Essentially performs (insert(std::make_pair(std::move(key), T())).first)->second

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 is returned.

Complexity

Logarithmic in the size of the container.

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)