How do I use macros in Perl, like I do in C?
Example:
#define value 100
print value;
I want to get the output as 100.
A:
Try the following:
macro.pl
use strict;
use warnings;
#define a 10;
print a;
And run this code using -P switch.
Example:
perl -P macro.pl
And use the module Filter::cpp also.
kiruthika
2010-03-26 04:46:48
From the perlrun man page: "NOTE: Use of -P is strongly discouraged because of its inherent problems, including poor portability."
Gavin Brock
2010-03-26 05:13:42
Has anyone ever found a good use for -P in production code?
Gavin Brock
2010-03-26 05:14:18
Although this technically answered the question, it would be better if it mentioned how messy things get when running a whole perl script through the C pre-processor with the -P flag. Indeed the perlrun manpage says in boldface "NOTE: Use of -P is strongly discouraged because of its inherent problems, including poor portability. It is deprecated and will be removed in a future version of Perl."http://perldoc.perl.org/perlrun.html
msw
2010-03-26 05:14:45
-1, -P is a horrible flag.
rjh
2010-03-26 11:57:01
@Gavin - doesn't "P" stand for "Production"? :)
DVK
2010-03-27 02:44:51
Downvoted. This is a *horrible* "accepted" answer.
Randal Schwartz
2010-09-08 02:23:30
+25
A:
If you just want to define constants (like your example) rather than full macros, there are a couple of perl ways to do this.
Some people like:
use constant value => 100;
print value;
Note that 'value' is a subroutine, not a 'variable'. This means you cannot interpolate it in strings so you have to do. print "The value is ".value."\n";.
The "Best Practices" crowd like:
use Readonly;
Readonly my $value => 100;
print $value;
However, unlike constant, Readonly is not part of the core perl distribution and so needs to be installed from CPAN.
Gavin Brock
2010-03-26 04:50:29
Constants are difficult to interpolate into a string, amongst other problems. Use **Readonly**.
Duncan
2010-03-26 06:15:03
Duncan: constants get optimized away if(someconst){...} will go away entirely during compilation if someconst is false. That is NOT the case for Readonly variables. Also, if you insist on using Readonly, ALWAYS install Readonly::XS!
tsee
2010-03-26 07:39:17
@tsee, sounds like micro-optimisation. I'm sure `if(0)` is fast enough; aside from that, the OP isn't asking for `#ifdef`. See http://search.cpan.org/perldoc?Readonly#COMPARISON_WITH_"use_constant" - use constant has tons of problems.
rjh
2010-03-26 12:00:42
@rjh: I disagree on most of the criticism in the Readonly documentation. Furthermore, the compile-time behaviour means that you also save the memory of the OP-structure of the code. Regarding if(0): You're wrong: With Readonly (at least the pure-perl version), you actually pay at least one subroutine call per access. That's *a lot* since subroutine calls are very slow. If you consider something like ''Readonly my $DEBUG'', and use it for debugging output liberally, you pay dearly at run-time.
tsee
2010-03-26 16:53:34
Constants aren't _that_ difficult to interpolate into strings. Thy might be slightly annoying, but not difficult.
brian d foy
2010-03-26 16:57:31
@tsee: I agree with your point about Readonly - XS is a given. Still, as a Moose user, I'm quite used to trading performance for more maintainable and less error-prone code.
rjh
2010-03-26 17:26:59
@brian - could you please elaborate? Did you mean http://dev.perl.org/perl6/rfc/252.html? perldoc constant explicitly states "Constants defined using this module cannot be interpolated into strings like variables" - and I don't recall any way interpolation would work on a subroutine. Thanks!
DVK
2010-03-27 02:53:26
Don't put too much stock in Perl 6 RFCs. Every crank came out to say all sorts of dumb things. It's not that difficult to write @{[CONSTANT]}, among the 10 other ways you could do the same thing.
brian d foy
2010-03-27 03:58:04
+8
A:
For constants, the common way is to use a constant subroutine that will get inlined by the compiler:
use constant value => 100;
or
sub value () {100}
Eric Strom
2010-03-26 04:52:35
+11
A:
Perl is not C. You would be much better served by learning the corresponding Perl idioms (such as the already mentioned use constant value => 100) than by trying to drag C idioms into Perl.
Dave Sherohman
2010-03-26 10:43:26