[cfe-users] Undefined behaviour in a small code snippet

[Łukasz Szczur via cfe-users]

 Hello,

The program at the bottom prints
1
2
1
2

Does the code contain undefined behaviour in C++17 (note that the structs
have common initial sequence )?

Łukasz


#include<iostream>

using namespace std;

struct A {
        int first;
        int second;
};

struct B {
        int first;
        int second;
};

struct Test {
        union {
                A a;
                B b;
        };
};

int main() {
        Test t;
        t.a.first = 1;
        t.b.second = 2;
        cout << t.a.first << '\n';
        cout << t.a.second << '\n';
        cout << t.b.first << '\n';
        cout << t.b.second << '\n';
        return 0;
}

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