Constant string reference

przemo has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

One way of making scalar read-only is to assign to glob the reference to constant scalar, e.g.

    *PI = \3.14;
[download]

However, I have a problem with constructing read-only strings. If

    *NAME = \"My name";
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is marked read-only,

    *NAME2 = \"My $name";
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isn't.

Bigger snippet:

    $bar = 'bar';

    # This prints 'bar'
    *foo = \"The $bar is here";
    $foo = "bar";
    print "$foo\n";

    # This dies
    *foo = \"The bar is here";
    $foo = "bar2";    # error here
    print "$foo\n";
[download]

My question is: how to make "non-constant" strings (e.g. variables) become constant ones? Neither concatenation with '.', nor variable embedding in string works for me.

Comment on Constant string reference Select or Download Code

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Re: Constant string reference by diotalevi (Canon) on Dec 09, 2004 at 12:58 UTC
This function is undocumented but comes with perl as of a 5.8.something version. `Internals::SvREADONLY( \ $variable, 1 )`	[reply] [d/l]
Re^2: Constant string reference by dave_the_m (Monsignor) on Dec 09, 2004 at 14:03 UTC
This function is undocumented but comes with perl as of a 5.8.something version. Internals:: functions are not supposed to be used by user-code outside the core; that's why they're undocumented. Dave.	[reply]
Re^3: Constant string reference by diotalevi (Canon) on Dec 09, 2004 at 14:50 UTC
No, that's not right. Internals::functions may be used by user-code but only by code that allows that perl internals are allowed to change from version to version. The B:: modules exist so that perl isn't a black box and to allow people access to the guts if they really need to be there.	[reply]
Re^4: Constant string reference by dave_the_m (Monsignor) on Dec 09, 2004 at 15:15 UTC
Re^5: Constant string reference by diotalevi (Canon) on Dec 09, 2004 at 15:17 UTC
Some notes below your chosen depth have not been shown here
Re: Constant string reference by reneeb (Chaplain) on Dec 09, 2004 at 12:48 UTC
use constant: `#! /usr/bin/perl use constant{PI => 3.14, STRING => 'Test'}; print PI,"\n",STRING,"\n";` [download]	[reply] [d/l]
Re^2: Constant string reference by przemo (Scribe) on Dec 09, 2004 at 13:00 UTC
I know I can use 'constant' pragma. However, I wanted to use plain scalar for some reasons.	[reply]
Re: Constant string reference by gellyfish (Monsignor) on Dec 09, 2004 at 12:56 UTC
~~There is a `readonly` subroutine in Scalar::Util that you can use to do this simply, or~~ If you have Inline available you can do this yourself trivially: `use Inline C; + my $FOO = "Bar"; + readonly($FOO); + $FOO = "GG"; __DATA__ __C__ + void readonly(SV *sv) { SvREADONLY_on(sv); }` [download] /J\	[reply] [d/l] [select]
Re^2: Constant string reference by Fletch (Bishop) on Dec 09, 2004 at 13:03 UTC
Erm, `Scalar::Util::readonly` reports but doesn't set the readonly flag: `readonly SCALAR Returns true if SCALAR is readonly. sub foo { readonly($_[0]) } $readonly = foo($bar); # false $readonly = foo(0); # true` [download]	[reply] [d/l] [select]
Re: Constant string reference by przemo (Scribe) on Sep 18, 2009 at 10:07 UTC
Just as chronicler's duty -- which I hope explains this self-response -- I'd add there are Readonly and Readonly::XS modules which would serve the OP's purpose.	[reply]