[ ] 自动插入map

performing an insertion if such key does not already exist.

#include <iostream>
#include <map>
 
int main()
{
    std::map<char, int> letter_counts;
 
    letter_counts['a'];
    for(auto& [k,v]: letter_counts) {
        std::cout<<k <<", "<<v<<std::endl; 
    }
    std::cout<<"-------------------"<<std::endl;
    letter_counts['b'];
    for(auto& [k,v]: letter_counts) {
        std::cout<<k <<", "<<v<<std::endl; 
    }
    std::cout<<"-------------------"<<std::endl;
    letter_counts['a']=1;
    for(auto& [k,v]: letter_counts) {
        std::cout<<k <<", "<<v<<std::endl; 
    }
    std::cout<<"-------------------"<<std::endl;
    letter_counts['a'];
    for(auto& [k,v]: letter_counts) {
        std::cout<<k <<", "<<v<<std::endl; 
    }
    std::cout<<"-------------------"<<std::endl;

    return 0;
}

output:

a, 0

-------------------

a, 0

b, 0

-------------------

a, 1

b, 0

-------------------

a, 1

b, 0

-------------------