Why doesn't 'd /= d' throw a division by zero exception when d == 0?

Question

I don't quite understand why I don't get a division by zero exception:

int d = 0;
d /= d;

I expected to get a division by zero exception but instead d == 1.

Why doesn't d /= d throw a division by zero exception when d == 0?

Answer 1

C++ does not have a "Division by Zero" Exception to catch. The behavior you're observing is the result of Compiler optimizations:

1. The compiler assumes Undefined Behavior doesn't happen
2. Division by Zero in C++ is undefined behavior
3. Therefore, code which can cause a Division by Zero is presumed to not do so.
   • And, code which must cause a Division by Zero is presumed to never happen
4. Therefore, the compiler deduces that because Undefined Behavior doesn't happen, then the conditions for Undefined Behavior in this code (d == 0) must not happen
5. Therefore, d / d must always equal 1.

However...

We can force the compiler to trigger a "real" division by zero with a minor tweak to your code.

volatile int d = 0;
d /= d; //What happens?

So now the question remains: now that we've basically forced the compiler to allow this to happen, what happens? It's undefined behavior—but we've now prevented the compiler from optimizing around this undefined behavior.

Mostly, it depends on the target environment. This will not trigger a software exception, but it can (depending on the target CPU) trigger a Hardware Exception (an Integer-Divide-by-Zero), which cannot be caught in the traditional manner a software exception can be caught. This is definitely the case for an x86 CPU, and most other (but not all!) architectures.

There are, however, methods of dealing with the hardware exception (if it occurs) instead of just letting the program crash: look at this post for some methods that might be applicable: Catching exception: divide by zero. Note they vary from compiler to compiler.

Answer 2

Just to complement the other answers, the fact that division by zero is undefined behavior means that the compiler is free to do anything in cases where it would happen:

• The compiler may assume that 0 / 0 == 1 and optimize accordingly. That's effectively what it appears to have done here.
• The compiler could also, if it wanted to, assume that 0 / 0 == 42 and set d to that value.
• The compiler could also decide that the value of d is indeterminate, and thus leave the variable uninitialized, so that its value will be whatever happened to be previously written into the memory allocated for it. Some of the unexpected values observed on other compilers in the comments may be caused by those compilers doing something like this.
• The compiler may also decide to abort the program or raise an exception whenever a division by zero occurs. Since, for this program, the compiler can determine that this will always happen, it can simply emit the code to raise the exception (or abort execution entirely) and treat the rest of the function as unreachable code.
• Instead of raising an exception when division by zero occurs, the compiler could also choose to stop the program and start a game of Solitaire instead. That also falls under the umbrella of "undefined behavior".
• In principle, the compiler could even issue code that caused the computer to explode whenever a division by zero occurs. There is nothing in the C++ standard that would forbid this. (For certain kinds of applications, like a missile flight controller, this might even be considered a desirable safety feature!)
• Furthermore, the standard explicitly allows undefined behavior to "time travel", so that the compiler may also do any of the things above (or anything else) before the division by zero happens. Basically, the standard allows the compiler to freely reorder operations as long as the observable behavior of the program is not changed — but even that last requirement is explicitly waived if executing the program would result in undefined behavior. So, in effect, the entire behavior of any program execution that would, at some point, trigger undefined behavior is undefined!
• As a consequence of the above, the compiler may also simply assume that undefined behavior does not happen, since one permissible behavior for a program that would behave in an undefined manner on some inputs is for it to simply behave as if the input had been something else. That is, even if the original value of d was not known at compile time, the compiler could still assume that it's never zero and optimize the code accordingly. In the particular case of the OP's code, this is effectively indistinguishable from the compiler just assuming that 0 / 0 == 1, but the compiler could also, for example, assume that the puts() in if (d == 0) puts("About to divide by zero!"); d /= d; never gets executed!

Answer 3

The behaviour of integer division by zero is undefined by the C++ standard. It is not required to throw an exception.

(Floating point division by zero is also undefined but IEEE754 defines it.)

Your compiler is optimising d /= d to, effectively d = 1 which is a reasonable choice to make. It's allowed to make this optimisation since it's allowed to assume there is no undefined behaviour in your code - that is d cannot possibly be zero.

Answer 4

The simplest way to understand what happens is to see the assembly output

int divide(int num) {
    return num/num;
}

will generate for x86-64

divide(int):
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-4], edi
        mov     eax, 1
        pop     rbp
        ret

As you can see there are no divide operations here, but we have mov eax, 1.

Here is a link to reproduce: https://godbolt.org/z/MbY6Wqh4T

Answer 5

Note that you can have your code generate a C++ exception in this (and other cases) by using boost safe numerics. https://github.com/boostorg/safe_numerics