Sequence-before relation in comma-separated variable definitions

Question

Let's begin with the following example code:

int a = 0, b = a++, c = a;

Is a++ sequenced before a (within c = a)? a++ and a seem to qualify as full expressions, and according to cppreference (Rule 1), the answer should be positive. But I'm not sure.

Answer 1

Yes. As Brian points out, this is not a comma operator, but rather an init-declarator-list. From [dcl.decl] we have:

Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.

With a footnote which clarifies:

A declaration with several declarators is usually equivalent to the corresponding sequence of declarations each with a single declarator. That is

T D1, D2, ... Dn;

is usually equivalent to

T D1; T D2; ... T Dn;

where T is a decl-specifier-seq and each Di is an init-declarator.

There are two exceptions, one for a name hiding a type and one for auto, neither of which apply. So ultimately, the code you have is exactly equivalent to:

int a = 0;
int b = a++;
int c = a;

Which you should prefer to write in the first place since it doesn't take searching through the standard to ensure that you're doing something valid!