Re: subset of a POD

James Kanze <>
Fri, 11 Dec 2009 02:01:14 -0800 (PST)
On Dec 11, 5:21 am, Victor Bazarov <> wrote:

James Kanze wrote:

On Dec 9, 7:15 pm, Victor Bazarov <> wrote:

Vladimir Jovic wrote:

Victor Bazarov wrote:

Hicham Mouline wrote:


"return" are set to some "invalid" value. As an
alternative you could consider supplying another member
to 'S', which would indicate the "valid" or "active"
members. You can have another (nested) struct in 'S'
with bit fields:

   struct S {
      ... // as you have now
      struct {
         unsigned use_1:1;
         unsigned use_2:1;
         unsigned use_n:1;
      } which_members_to_use;

Using "bool use_1" might be clearer then "unsigned use_1:1".

Clearer, maybe. More expensive, for sure.

More expensive in memory. Probably significantly less expensive
in runtime.

Curious, isn't it, how one can easily tell when it's more
expensive from the memory standpoint, and how unknown the
expense in terms of the run time is. We can't tell whether
it's "significantly less expensive" until we actually measure,
can we? <g>

Technically no, and the "more expensive" also depends on what
you're doing with the object. Which is why I said
"probably":-). Still, none of the machines I know can
physically access less than a byte at a time, so runtime access
to the bit fields requires additional masking and possibly
shifting, and modifying them requires a read before hand in
order to avoid modifying neighboring bitfields.

The next version of the standard will recognize this, at least
partially---accessing different bitfields in a single struct is
considered accessing a single object with regards to threading,

    struct S1 { bool b1 ; bool b2 ; } s1;
    struct S2 { bool b1:1; bool b2:1; } s2;

Given the above, one thread can use s1.b1, and another s1.b2,
without any external synchronization; the same scenario for s2
is undefined behavior.

James Kanze

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