C++: variable declaration initialization order

Question

When I'm defining some variables like this:

int a = pop(), b = pop(), c = pop();

does C++ give a guarantee that a is going to be initialized first, then b and then c? or is the order not defined?

Could you explain why this is important? We might give a better answer — alestanis
– alestanis, Commented Mar 6, 2013 at 13:29
Improve readability and assert your order by using 3 lines. It's as simple as that. What's the point of getting rid of two lousy lines anyway? — stefan
– stefan, Commented Mar 6, 2013 at 13:35
I agree with @stefan. I rarely (never?) use multiple declarations in one line. — alestanis
– alestanis, Commented Mar 6, 2013 at 13:46
If this occurs in a for-init-statement such as for (int a = pop(), b = pop(), c = pop(); ...) then the question is more interesting. You could put two of the declarations on the previous line, but now they have a different scope — Jonathan Wakely
– Jonathan Wakely, Commented Mar 6, 2013 at 13:47

Jonathan Wakely · Accepted Answer · 2013-03-06 13:30:44Z

12

[dcl.decl]/3 says

-3- Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.

Which means your code is treated like:

int a = pop();
int b = pop();
int c = pop();

answered Mar 6, 2013 at 13:30

Jonathan Wakely

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7 Comments

Paul R Over a year ago

Hmm - I'm not convinced that this implies a predictable order of initialisation...

user1773602 Over a year ago

which still doesn't guarantee any particular order.

Andy Prowl Over a year ago

Note 97 provides an explanation, but uses the work "usually". I believe, however, that this only means that there are well-defined exceptions to this rule, but it is not "up to the compiler".

Steve Jessop Over a year ago

@JonathanWakely: so arguably a (minor) defect in the standard, if one has to rely on a footnote for a normative statement of the required behavior. But I think you're correct about what the standard intends to define :-) In such moments I usually say that although footnotes aren't normative, they generally are true.

quetzalcoatl Over a year ago

@JonathanWakely: yes, I've just noticed that. [dcl.init] is exactly the place, and in fact in the note #97 (open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf) it is quite clearly stated that the T a,b,c is a "sequence" and is equivalent to T a;T b;T c;. The word "usually" refers not to exception in the comprehension of a sequence, but to the potential collision of identifiers: T a, T, b, c; is surely not equivalent to T a; T T; T b; T c;. Therefore I agree that it is defined, but damn, I'd like it to be written plainly as a proper point, not a footnote..

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