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# Copyright (C) 2007-2012, Parrot Foundation.
=pod
=head1 DESCRIPTION
A tutorial lesson about Parrot's defined opcode.
=head1 DEFINED
The C<defined> opcode tells you if the contents of a PMC is defined or not.
Using C<defined> on a C<int>, C<num>, or C<string> register may generate
an error if the register has been used before and freed or is newly created.
C<defined> is a great way to test a PMC to ensure it's been set to a proper
value before attempting to use it. Attempting to use a PMC that has not
been defined may throw an exception or cause a bigger problem.
=cut
.sub main :main
$P1 = new ['String']
$I0 = defined $P1
if $I0 goto defined_P1
say "$P1 is undefined"
goto end_defined_P1
defined_P1:
say "$P1 is defined"
=pod
Most PMC's, but not all, should return true for C<defined>. It all
depends on how the PMC implements its vtable function for C<defined>.
For example the C<Undef> PMC always returns false (0) for C<defined>.
=cut
end_defined_P1:
$P3 = new ['Undef']
$I0 = defined $P3
if $I0 goto defined_P3
say "$P3 is undefined"
goto end_defined_P3
defined_P3:
say "$P3 is defined"
end_defined_P3:
.end
# Local Variables:
# mode: pir
# fill-column: 100
# End:
# vim: expandtab shiftwidth=4 ft=pir:
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