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

[Matthew Fernandez via cfe-users]

I don’t have the reference on hand, but I’m pretty sure writing to one union member and reading the value back through another is undefined behaviour. I believe this is inherited from C99 or earlier. 

> On 19 Mar 2018, at 12:00, via cfe-users <cfe-users at lists.llvm.org> wrote:
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>   1. Undefined behaviour in a small code snippet
>      (Łukasz Szczur via cfe-users)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Tue, 13 Mar 2018 01:14:51 +0100
> From: Łukasz Szczur via cfe-users <cfe-users at lists.llvm.org>
> To: cfe-users at lists.llvm.org
> Subject: [cfe-users] Undefined behaviour in a small code snippet
> Message-ID:
>    <CADW1ASR2he1C2YD+v8bE5yysgFMFi7jzCoszW2H_VYM2EBNPdg at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> 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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