Hi there,
I intended to write a little module that would make a Perl variable
read-only (without the help of tie() of course). I thought this would be
easy, but not so obviously! I am using Attribute::Handlers to allow such
a thing:
my $var : Readonly("I'm immutable");
In Readonly.pm:
sub Readonly :ATTR(SCALAR) {
my ($var, $data) = @_[2, 4];
# set value than make it readonly via XS
$$var = $data;
_ro_scalar($$var);
}
In Readonly.xs:
void
_ro_scalar (var)
SV * var;
PREINIT:
struct ufuncs uf;
MAGIC *m;
CODE:
uf.uf_val = &get_val;
uf.uf_set = &set_val;
uf.uf_index = 0;
sv_magic(var, 0, 'U', (char*)&uf, sizeof(uf));
//--------> /* valus has been set: now change magic to let svt_set() croak */
m = mg_find(var, 'U');
if (m == NULL)
printf("no magic\n");
else /* redefine */
m->mg_virtual->svt_set = &croak_set_val;
set_val is functionally a no-op:
I32 set_val (IV i, SV* s) {
printf("SETTING: %i - %s\n", i, SvPV_nolen(s));
return 1;
}
croak_set_val:
int croak_set_val (SV *s, MAGIC *mg) {
Perl_croak("\nAttempt to set read-only variable");
return 1;
}
If the part on from '//--------->' in _ro_scalar() is not there,
set_val() is already called from 'Readonly :ATTR(SCALAR)'. So basically
I need two different svt_set function pointers: one for the initial
setting of the value in Readonly.pm:
$$var = $data;
and one later (croak_set_val()) when the variable should really be
read-only. Why is this so? In the following code:
$$var = $data;
_ro_scalar($$var);
I would assume that the value is first set (which should be no problem
since no magic has yet been attached to $$var) and then any attempt to
re-set the value should invoke set_val. But to me it appears as though
_ro_scalar() is invoked before assigning $data to $$var.
My current workaround is to change mg_virtual->svt_set from &set_val()
to croak_set_val() which works fine. But why do I have to do this?
I also tried it with 'Reload :ATTR(SCALAR, BEGIN)' to let it happen at
compile-time, but to no avail.
Can anyone help?
Tassilo
--
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