On 27 Nov 2009, at 13:30, Jeremy Pereira wrote:
> You can't have non constant elements in struct initialisers, testy is not a
> constant. The error message spells out exactly what the issue is.
You *can* have non-constant elements in struct initialisers in C, *but* only in
a context where there is code that will execute at runtime.
So
struct foo {
int foo;
};
int fn (int anInt) {
struct foo fooArray[] = { { anInt }, { anInt + 1 } };
}
is quite legitimate, while as you rightly point out
extern int anInt;
struct foo {
int foo;
}
struct foo fooArray[] = { { anInt }, { anInt + 1 } };
isn't.
In the case of C89/C90, this is an extension provided by GNU C and by some
other compilers. It's a formal part of C99, however (see §6.7.8 ¶4 - "All the
expressions in an initializer for an object that has static storage duration
shall be constant expressions or string literals.").
In C++, however, the standard says that it's allowed wherever, including for
objects that have static storage (see §8.5 ¶2), so in C++, both of those pieces
of code should compile. There are some caveats, however, in that the order of
initialisation of objects is not specified; the standard does guarantee that,
if they haven't yet been initialised dynamically, they'll have been temporarily
initialised to zeroes (§8.5 ¶6). Given that there are additionally some issues
surrounding the implementation of this feature (e.g. it may be necessary to
call a special function to perform these initialisations on some platforms if
the module in question doesn't have a conventional main() function, for
whatever reason), it's probably best to avoid relying on this ability.
Kind regards,
Alastair.
--
http://alastairs-place.net
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