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

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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.

### 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 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)

### 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
--- | --- | --- | ---
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

### See also

at | 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)

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