std::map::operator[] - cppreference.com

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# std::map::operator[]

From cppreference.com
< cpp‎ | container‎ | map

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.

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 value_type(key, T())

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

2) Inserts value_type(std::move(key), T())

- `key_type` must meet the requirements of `MoveConstructible`. (since C++11)
- `mapped_type` must meet the requirements of `DefaultConstructible`. (since C++11)

No iterators or references are invalidated.

### [edit] Parameters

key | - | the key of the element to find

### [edit] 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`.

### [edit] Exceptions

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

### [edit] Complexity

Logarithmic in the size of the container.

### Notes

Until C++11, the overload (1) was specified to be equivalent to (insert(std::make_pair(key, T())).first)->second, which required T to be CopyConstructible.

### [edit] 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'

### [edit] See also

at (C++11) | access specified element with bounds checking (public member function) [edit]

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