How do languages handle side effects of compound operators?

Assume such situation:

int a = (--t)*(t-2);
int b 开发者_JAVA百科= (t/=a)+t;

In C and C++ this is undefined behaviour, as described here: Undefined behavior and sequence points

However, how does this situation look in:

• JavaScript,
• Java,
• PHP...
• C#
• well, any other language which has compound operators?

I'm bugfixing a Javascript -> C++ port right now in which this got unnoticed in many places. I'd like to know how other languages generally handle this... Leaving the order undefined is somehow specific to C and C++, isn't it?

According to the ECMA Script specification, which I believe javascript is supposed to conform to, in multiplication and addition statements, it evaluates the left hand side before evaluating the right hand side. (see 11.5 and 11.6). I think this means that the code should be equivalent to

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

However, you should not always trust the specification to be the same as the implementation!

Your best bet in confusing cases like this is to experiment with various inputs to the ambiguous lines of code in the original operating environment, and try to determine what it is doing. Make sure to test cases that can confirm a hypothesis, and also test cases that can falsify it.

Edit: Apparently most JavaScript implements the 3rd edition of ECMAScript, so I changed the link to that specification instead.

Practically speaking, if you have to ask or look up the rules for an expression, you shouldn't be using that expression in your code. Someone else will come back two years from now and get it wrong, then rewrite it and break the code.

If this was intended as a strictly theoretical question I unfortunately can't offer details regarding those other languages.

For javascript the following article should help.

This article clearly states whether a particular combination of

a OP b OP c goes from left-to-right and in which order.

I'm don't know about the other languages.

However, how does this situation look in: JS, Java, PHP, C#...

To be perfectly candid, int a = (--t)*(t-2); int b = (t/=a)+t; looks like crap.

It's nice to have fancy code that can be all pretty and elitist, but there's absolutely no need for it. The solution for every language when confronted with code like this is to add a couple more semi-colons (unless you're dealing with python):

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

If a different order of operations is desired, the adjust the lines accordingly. If you're trying to fix old buggy code, fix the code, don't just re-implement someone else's spaghetti.

Edit to add:

I realized I never specifically answered the original question:

How do languages handle side effects of compound operators?

Poorly.