CODEWITHSUNDEEP
c
a06e
a06e

Reputation: 20774

Does += return the value of the left hand side after assignment?

What does the C standard (preferably C89,90) say about:

int a,b;
a = 4;
b = (a += 1);

?

I have tested it and the result is b=5, which is what I expect. I just want to be reassured by the Standard. The same applies to analogous operators like *=, /=, &=, etc. I know that = is sure to return the value of the left hand side after the assignment. I just want to know if +=, *=, etc. behave the same way, according to the standard.

Upvotes: 1

Views: 1168

Answers (3)

LNO
LNO

Reputation: 387

This should result in b = 5 and a = 5. This is because a+=1 takes a and adds 1 to it. Then it assigns a to b.

I'm kind of confused on your question, but *=, /=, -= all work the same way.

For example, you could just have int c = 7; then on the next line do c*=3. This will make c = 21. If in your example, you didn't want a = 5; then just don't do a+=1, and instead, do a+1.

Upvotes: 0

eq-
eq-

Reputation: 10096

Assignment operators do not "return" a value: they yield one, or as the standard puts it, have one.

The value is of the left operand, although it won't be an lvalue. Here's the excerpt:

(3.3.16) An assignment expression has the value of the left operand after the assignment, but is not an lvalue.

All of = *= /= %= += -= <<= >>= &= ^= |= are assignment operators so they behave the same way in this regard.

Upvotes: 4

ronalchn
ronalchn

Reputation: 12335

It is not a problem unless there are side-effects.

The assignment operator is not a sequence point, which means that there is no guarantee on order of evaluation.

If you use it as you have given (b = (a += 1);), it is not a problem.

However, in other cases, it may be a problem, for example:

b = (a += 1) + a; // undefined

Notice that in this example, the variable a is referred to twice. What that means is that we don't know whether (a += 1) or a is evaluated first. So we don't know if the 2nd reference to a will be before or after 1 was added to it.

If you only refer to each variable you assign with += and co. once, then side-effects are not a problem, and you can count on += and related operators to return the value after assignment.

Upvotes: 1

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