Re: In-memory order of class variables

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Fri, 21 Nov 2008 07:19:54 CST
On 21 Nov., 02:34, Alberto Ganesh Barbati <>

irotas ha scritto:

Consider the following class:
class Foo
  bool a;
  int b;
  char c;

Is it always, no matter what, infallibly true, that Foo::a will appear
in memory *before* Foo::b, and Foo::b before Foo::c? (I'm talking
about userland here, not kernel)

I always assumed that to be true, but someone told me recently that
they thought that some architectures might reorder the variables in
memory (if the compiler is configured to pack struct's) in order to
obtain an optimal memory footprint.

This "someone" is wrong. 9.2/12 says that explicitly:


So the only reordering allowed is due to the presence of
access-specifiers. For example an implementation may decide store all
private members before all public members, regardless of the declaration
order. For what it's worth, I'm not aware of any implementation that
exploits this possibility.

What you say is right regarding the OP's example, but to be fair, the
OP is right that the current official standard 14882:2003(E) has one
further saying to that in 11.1 [class.access.spec]/2:

"The order of allocation of data members with separate access-
labels is unspecified (9.2)."

which is probably the reason for the "someone" to mention that point.
It could happen (according to the old wording), if you define Foo
seemingly equivalent as

class Foo
  bool a;
  int b;
  char c;

Both paragraphs have been fixed regarding this situations as part of

11.1/2 has been removed and now the draft says in 9.2/12

"Nonstatic data members of a (non-union) class **with the same access
control** (Clause 11) are allocated so [..]"

instead of

"Nonstatic data members of a (non-union) class declared **without an
intervening access-specifier** are allocated so [..]"

(Emphasis mine)

- Daniel

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