c++ – structure in map allocation

February 3, 2022

As I known, map value is initialized by NULL(0). However, Below code is works well without any allocation. How is this code work?

#include<bits/stdc++.h>
using namespace std;
struct stuc{
    map<string, stuc> mp;
    int cnt;
}root;

int main() {
    stuc* u = &root;
    stuc* v = &(u->mp["test"]); // how to allocate?
    cout << v->cnt << endl;
    return 0;
}

>Solution :

If you access a map with the [] operator like in

&(u->mp["test"])

there is a new key value pair allocated, default initialized, and inserted into the map if the element does not already exists [1]. In your program that is exactly what happens. The map does not already have an key "test". Thus a new value is default constructed. Because you did not define a custom default constructor for your stuc struct, all its members are default initialized (mp = {} and cnt = 0). This is the reason why you get 0 if you read the cnt member of the newly created value.

[1] https://en.cppreference.com/w/cpp/container/map/operator_at