The limitations of the comma operator

Question

I have read this question and I want to add to it that what are the things that can not be done using the comma operator. This has confused me a lot, as I can do this:

int arr[3];
arr[0]=1,arr[1]=2,arr[2]=3;

But when I do:

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

It gives me a compiler error.

I want to ask that what are the limitations of the comma operator in real practice?

Answer 1

One thing to realize is that not all uses of a comma in C are instances of the comma operator. Changing your second example to be a syntactically declaration:

int a0=1,a1=2,a2=3;

the commas are not operators, they're just syntax required to separate instances of declarators in a list.

Also, the comma used in parameter/argument lists is not the comma operator.

In my opinion the use of the comma operator is almost always a bad idea - it just causes needless confusion. In most cases, what's done using a comma operator can be more clearly done using separate statements.

Two exceptions that come to mind easily are inside the control clauses of a for statement, and in macros that absolutely need to cram more than one 'thing' into a single expression, and even this should only be done when there's no other reasonable option).

Answer 2

You can use the comma operator most anywhere that an expression can appear. There are a few exceptions; notably, you cannot use the comma operator in a constant expression.

You also have to be careful when using the comma operator where the comma is also used as a separator, for example, when calling functions you must use parentheses to group the comma expression:

void f(int, bool);

f(42, 32, true);   // wrong
f((42, 32), true); // right (if such a thing can be considered "right")

Your example is a declaration:

int arr[3],arr[0]=1,arr[1]=2,arr[2]=3;

In a declaration, you can declare multiple things by separating them with the comma, so here too the comma is used as a separator. Also, you can't just tack on an expression to the end of a declaration like this. (Note that you can get the desired result by using int arr[3] = { 1, 2, 3 };).