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";
}

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'