I cant seem to understand how this code works, I know it might be really easy but I'm kinda a noob

[silver prairie]

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@torn condor

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[torn condor]

#include <iostream>
using namespace std;
int main ()
{
int x, y;
x = 5;
y = ++x * ++x;
cout << x << y;
x = 5;
y = x++ * ++x;
cout << x << y;
return 0;
}

[radiant scroll]

this code has undefined behavior.

[torn condor]

wdym?

[torn condor]

.

[arctic hatch]

that the result of it is undefined

y = x++ * ++x;
stuff like this is also just not great

[torn condor]

no i added it in a c++ ide it gives answer 749735

[arctic hatch]

thats not what undefined means

[torn condor]

i'm really sorry but i'm really a beginner at this

i really dont understand

[arctic hatch]

undefined means that the result could be whatever

or atleast are undetermanistic

[torn condor]

yh but take for example this line

int x, y;
x = 5;
y = ++x * ++x;

what should the value of y be?

[arctic hatch]

I believe that one is mostly fine, it's the one later that's problematic

[torn condor]

yh but what could y be?

[arctic hatch]

afaik this one is just 6 * 7 or 7 *6

[torn condor]

yh but it gives 49 as answer

[arctic hatch]

;compile

#include <iostream>
using namespace std;
int main ()
{
int x, y;
x = 5;
y = ++x * ++x;
cout << "X:" << x  << " "<< "Y:" <<  y << "\n";
x = 5;
y = x++ * ++x;
cout << "X:" << x  << " "<< "Y:" <<  y << "\n";
return 0;
}

[leaden spear]

Program Output

X:7 Y:49
X:7 Y:35

mcmlevi#0000 | 46ms | c++ | x86-64 gcc 13.2 | godbolt.org

[torn condor]

you see

[arctic hatch]

the result isn't 749735.....

[torn condor]

no but for the first part x becomes 7 and y becomes 49 ryt?

you said 42 i thought 42 too

[arctic hatch]

🤔 right no it would make sense if the final result was 7 for both of them

anyhow doing shit like this on 1 line is asking for trouble cause of exactly this reason

actually thinking about it it makes a lot of sense 🤔 and my assumption was propably wrong

[torn condor]

could you explain it to me please?

@arctic hatch you there?

[arctic hatch]

The simplest solution is simply don't write confusing code like this and it's easy to reason about

[radiant scroll]

that's the point, there's no value that y should be here.

the behavior of this code is undefined.

[torn condor]

wtf i didn't write this

its a question i got as homework as an mcq

[arctic hatch]

Then just awnsers the end result is UB :P

[torn condor]

what kind of reply is this

[arctic hatch]

I didn't mean with that you wrote it

[torn condor]

so you dont know?

[arctic hatch]

No I'm saying you can't know

Cause it's undefined behavior

[torn condor]

bruv

[arctic hatch]

But all I was saying with that is by simply not writing confusing code like this you solve the problem of ambiguity

[torn condor]

i'm saying why did y have a value of 49 instead of 42

[arctic hatch]

Where is this hostility coming from

[torn condor]

i'm asking you a question you're not answering properly

[arctic hatch]

It's as dot was saying, there is no guaranteed value Y is supposed to be

[torn condor]

how tf

what about the prefix increment

why did it become 49

[radiant scroll]

again, this code has undefined behavior

[torn condor]

x = 5;
y = ++x * ++x;

look at this part

[arctic hatch]

Because it propably evaluated x to be 7

[arctic hatch]

We are

[torn condor]

what value should y be

[radiant scroll]

it's undefined

[arctic hatch]

You do not know, it's undefined

[torn condor]

how so????

[radiant scroll]

because the rules of C++ say so

[arctic hatch]

It's undefined innit cause you can't know what the value of X it will use to evaluate it right? So 7x7, 6x7 and 7x6 be equally valid no?

[torn condor]

i have an mcq question telling me this lines of codes and has choices

you want me to say undefined??

[arctic hatch]

Then the question is wrong most likely

[brazen bear]

there's no correct answer

[torn condor]

wait but hasn't x been assigned a value of 5

y should computate using that value ryt?

[brazen bear]

no...

[torn condor]

why

[radiant scroll]

basically, it's undefined in which order operands are evaluated. in your example, it's undefined at which points and in which order during evaluation of the expression the side effects of modifying the value of x take place.

[torn condor]

yh but when compiled it gave an answer

[arctic hatch]

Yeah, thats what undefined does

Say you have a million options when you compile it gives you one of them

[silver prairie]

Undefined Behavior

Undefined behavior occurs when you violate rules specified by the language. For example: Reading uninitialized memory, performing out-of-bounds memory access, signed integer overflow, and race conditions.

Example: Read Indeterminate Val.

int i; // default init,
       // indeterminate value
while(i < 10) {
    printf("%d\n", i++);
}

Example: Out-of-Bounds Access

int arr[10];
for(int i = 0; i < 20; i++) {
    arr[i] = i; // [i] out of
                // bounds
}

Consequences of UB

Compilers are not required to provide warnings or errors about UB. Often it is undetectable at compile-time. Performing actions which are UB can render your entire program's behavior undefined, leading to anything from crashing to summoning Eldritch Abominations.

See Also

• cppreference: Undefined Behavior
• What is Undefined Behavior?

Sanitizers can help identify UB. For more info, look into address sanitizer (ASan) and undefined behavior sanitizer (UBSan). See !howto asan

[arctic hatch]

It seems like the compiler ended up making it 7*7

[brazen bear]

++x increments x.
if you wrote x = 5; y = ++x; then both x and y would be 6
but
++x * ++x increments x twice without a sequence point
so it's undefined

[torn condor]

ah ok

[arctic hatch]

So afaik it can be, 7x7, 6x7 or 7x6

Hence why I said simply don't do this

[azure nimbus]

Or it can print "Lol, fix your program bro"

[torn condor]

ok i'm getting it now

[radiant scroll]

it's ub, so it doesn't have to be anything.

the compiler could also just optimize away your whole code

[arctic hatch]

Fair, though I'd not expect it to go to far of the rails here

[torn condor]

🙏 really sorry for being aggressive and insulting you

[brazen bear]

it could also be 6x6 because the two reads and writes are unsequenced

[torn condor]

i'm really sorry i'm new at c++ and really confused

[arctic hatch]

Yeah all good

But yeah the entire reason I said don't so this, is cause stuff like this becomes a minefield

[torn condor]

ok

[arctic hatch]

Blegh I swear c++ sometimes

[radiant scroll]

it could also be 42×-1

[radiant scroll]

most-likely, it would just not be anything the moment you turn optimizations on

[arctic hatch]

Yeah, fair

[brazen bear]

;asm -O

int a (int x) {
  return ++x * ++x;
}
int b (int x) {
  auto inx = [&] { return ++x; };
  return inx() * inx();
}

[leaden spear]

Assembly Output

a(int):
  lea eax, [rdi+2]
  imul eax, eax
  ret
b(int):
  lea eax, [rdi+2]
  add edi, 1
  imul eax, edi
  ret

abcq2#0000 | 58ms | c++ | x86-64 gcc 13.2 | godbolt.org

[brazen bear]

the difference between unsequenced and indeterminately sequenced

[radiant scroll]

;asm cclang_trunk -O3

int main()
{
    int x = 1;
    return ++x * ++x;
}

[leaden spear]

Assembly Output

main: # @main
  mov eax, 6
  ret

dot42#0000 | 262ms | cclang_trunk | x86-64 clang (trunk) | godbolt.org

[brazen bear]

i think it's still UB if you make it volatile

cclang_trunk

[radiant scroll]

oh well

[radiant scroll]

yeah ofc

clang is normally pretty aggressive with treating ub as unreachable so I'd have expected it to do that here