c++ - structure in map allocation

Question

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;
}

What do you mean by "without any allocation"? `std::map` handles dynamic allocations internally — UnholySheep, Feb 03 '22 at 13:37
If this part is confusing: `stuc* u = &root;` the `root` variable is a global variable. — drescherjm, Feb 03 '22 at 13:38
OT but good reading: https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h — Jabberwocky, Feb 03 '22 at 13:41
_"...map value is initialized by NULL(0)"_: this does not make any sense. — Jabberwocky, Feb 03 '22 at 13:42

score 3 · Accepted Answer · answered Feb 03 '22 at 13:55

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

Odysseus · Answer 2 · 2022-02-03T14:09:48.917

When accessing a key from your std::map which does not yet exist, it is inserted.

So by accessing mp["test"] a default constructed instance of struct stuc gets inserted for key "test"

Consider the following example where also a value is implicitly inserted when accessing a non-existing key and its value being set by your default constructor

 struct stuc{
    stuc(): cnt(42) { }
    stuc(int c): cnt(c) { }
    int cnt;
};

int main() {
    map<string, struct stuc> m;

    struct stuc s1(1);

    m["first"] = s1;
    cout << m["first"].cnt << endl;
    cout << "elements: " << m.size() << endl;
    cout << m["second"].cnt << endl;
    cout << "elements: " << m.size() << endl;
    return 0;
}

which will output:

1
elements: 1
42
elements: 2

c++ - structure in map allocation

2 Answers2