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

#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";
}

Вывод:

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'