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

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.

# Declarations

T& operator[]( const Key& key );

(since C++23)

T& operator[]( Key&& key );

(since C++23)

template< class K >
T& operator[]( K&& x );

(since C++23)

# Parameters

# 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.

# Example

#include <iostream>
#include <string>
#include <flat_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::flat_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::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";
}

# See also