What methods do languages use to manage the side effects caused by compound operators?

Let's consider a scenario like this:

int a = (--t)*(t-2);
int b = (t/=a)+t;

While in C and C++, this leads to undefined behavior, as explained in this article: Undefined behavior and sequence points

But how is this situation handled in:

  • JavaScript,
  • Java,
  • PHP...
  • C#
  • or any other language that uses compound operators?

I'm currently fixing a bug in a Javascript -> C++ conversion project where this issue was overlooked in multiple places. It would be interesting to know how other languages generally address this... Is leaving the order undefined a specific feature of C and C++?

javascriptc++cprogramming-languagesundefined-behavior

Answer №1

As per the ECMA Script guidelines, which outline the expectations for JavaScript compliance, when it comes to multiplication and addition operations, the left side is evaluated before the right side (refer to sections 11.5 and 11.6). This suggests that the following code snippet should produce the same result:

t = t - 1;
int a = t * (t - 2);
t = t / a;
int b = t + t;

Nevertheless, it's important to note that the specification might not always align perfectly with the actual implementation!

In cases where things seem unclear, the best approach is to experiment with different inputs for the uncertain code lines in the original environment and analyze the behavior. Testing scenarios that can support or disprove a hypothesis is crucial.

Update: It appears that the majority of JavaScript follows the 3rd edition of ECMAScript, so I have directed the link to that specific version of the specification.

Answer №2

Simply put, if you find yourself needing to consult the rules for an expression, it's best to avoid using that expression in your code. It's likely that someone else will come across it in the future, misunderstand it, make changes, and unintentionally introduce bugs.

As for theoretical questions, I regret to inform you that I am unable to provide information on languages other than the one being discussed.

Answer №3

If you need assistance with JavaScript, you may find the information in this article helpful.

This resource clearly explains the order in which a sequence like a OP b OP c is executed, whether it is from left to right or in a different order.

I cannot speak for other programming languages when it comes to operator precedence.

Answer №4

But what about this scenario in languages like JS, Java, PHP, C#...

To be completely honest, the code snippet 

int a = (--t)*(t-2); int b = (t/=a)+t;
appears quite messy.

While having elegant and sophisticated code may seem appealing, it's often unnecessary. The remedy for dealing with such code in any programming language is as simple as adding a few more semi-colons (except for Python):

--t;
int a = t * (t-2);
t /= a;
int b = t + t;
-or-
int b = t * 2;
-or-
int b = t << 1;
//use whichever method you prefer

If you wish to change the order of operations, feel free to rearrange the lines accordingly. When fixing faulty old code, it's crucial to actually fix the code instead of just replicating someone else's messy structure.

Additionally:

I realize that I didn't directly address the initial question:

How do programming languages manage the side effects of compound operators?

Not very effectively.

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