C++ Order of Declaration (in Multi-variable Declaration Line)

Question

I use the following in my C++ code:

int a = 0, b = a;

I would like to know if this behaviour is reliable and well defined (left to right order of name declaration) and that my code will not break with other compilers with an undeclared name error.

If not reliable, I would break the statement:

int a = 0; 
int b = a;

Thank you.

Answer 1

I believe the answer is no.

It is subject to core active issue 1342 which says:

It is not clear what, if anything, in the existing specification requires that the initialization of multiple init-declarators within a single declaration be performed in declaration order.

We have non-normative note in [dcl.decl]p3 which says:

...[ Note: 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;

...

but it is non-normative and it does not cover the initialization case at all and as far as I can tell no normative wording says the same thing.

Although the standard does cover the scope of names in [basic.scope.pdecl]p1 which says:

The point of declaration for a name is immediately after its complete declarator and before its initializer (if any), except as noted below. [ Example:

unsigned char x = 12;
{ unsigned char x = x; }

Here the second x is initialized with its own (indeterminate) value. — end example  ]