[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

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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Re: [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  wrote:
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>   1. Undefined behaviour in a small code snippet
>  (Łukasz Szczur via cfe-users)
> 
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> --
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> Message: 1
> Date: Tue, 13 Mar 2018 01:14:51 +0100
> From: Łukasz Szczur via cfe-users 
> To: cfe-users@lists.llvm.org
> Subject: [cfe-users] Undefined behaviour in a small code snippet
> Message-ID:
>
> 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
> 
> 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;
> }
> -- next part --
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