Overriding built-ins

[chas]

I thought I could override a built-in by using the subs pragma as in:

use strict;
use warnings;
use subs qw(print);
sub print {CORE::print "OK, $_[0]\n";}
print "bla";

However, the result is just to print "bla". I wonder why the print function wasn't overridden. (BTW, if I replace the 1st, 2nd, and 4th occurrences of "print" with "shift", i.e. everywhere except in "CORE::print", then the shift function is overridden.) Thanks for any comments!
chas

[tlm]

You may find this node useful.

The short answer is that print is one of a handful of builtins that cannot be overridden. See in particular diotalevi's comments in the thread referred to above.

[chas]

Thanks tlm (and everyone else)! I should have searched perlmonks before asking (I did look in the docs but didn't find the answer.) The print function is an obvious choice to try out overriding since it would be easy to see the effect; presumably one wouldn't actually want to do this (for other than experimentation), though.
chas

[dragonchild]

print is one of those non-overridable built-ins. There's about a half-dozen or so that can't be overriden. *shrugs*

[Roy Johnson]

You can't override print.

[ikegami]

Not all core functions can be overridden. print is one of those that cannot.

Even the following doesn't work

package MyPrint;

sub print {
   shift;
   CORE::print("[\n");
   CORE::print(@_);
   CORE::print("]\n");
}

package main;

bless *STDOUT{IO}, 'MyPrint';
print("Hello World!\n");            # builtin
print STDOUT ("Hello World!\n");    # builtin

but this does

STDOUT->print("Hello World!\n");    # custom

I think tieing also works.

[Joost]

I don't know if you can override print: perlsub says:

... some built-ins (e.g. "exists" or "grep") can’t be overridden.

[ysth]

Usually you want to use the same prototype for an overriding function as the builtin has. If prototype("CORE::foo") is not defined, foo is usually not overridable (though require is an exception).

[diotalevi]

Other people linked to my old node at Re: !Overriding Builtin print. Initial Devel::UncheckedOps, a macro for perl presents a way that print() and other unoverridables can be be overriden *anyway*.