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std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::operator[]

From cppreference.com
< cpp‎ | container‎ | flat map
 
 
 
 
T& operator[]( const Key& key );
(1) (since C++23)
T& operator[]( Key&& key );
(2) (since C++23)
template< class K >
T& operator[]( K&& x );
(3) (since C++23)

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 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 try_emplace(x).first->second;.
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 try_emplace(std::move(x)).first->second;
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.

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, plus the cost of insertion (if any) of an empty element.

[edit] Notes

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.

[edit] Example

#include <iostream>
#include <string>
#include <flat_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::flat_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::flat_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'

[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]