velVerlet1 has asked for the wisdom of the Perl Monks concerning the following question:

Ciao Monks:

Newbie here, and trying to do a somewhat ambitious project, so apologies if this is somewhat stupid.
I'm trying to create a perl interface to my C++ finite element code.
The basic structure of the code is as follows:
I have an initiator function
void init(int argc, char **argv, void **ptr);
which creates a new instance of my class and assigns a pointer to it (ptr)

I have several member functions such as
int version(void *ptr);
which returns the various properties i'm interested in (in this case, ptr->version)

I've created a shared C++ library (mylib.so) already.

In order to create the perl interface, I used SWIG with the following interface file:

%include "exception.i"
%exception {
    try {
        $action
        }
    catch (const std::exception &e) {
        SWIG_exception_fail(SWIG_RuntimeError, e.what());
    }
}
%module mylib
%{
/* Put headers and other declarations here */
#include "library.h"
%}

%include "std_vector.i"
%include "std_string.i"

%include "library.h"

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Then I run swig and compile, which all goes well.
My question:
I want to do something like the following in perl

my $control = new mylib;
print "version: " . $control->version() . "\n";

However, whenever I try to access any of my member functions, the code complains:

TypeError in method 'version', argument 1 of type 'void *' at ./test_swig.pl line 8.

So basically, the function version expects a pointer (to self I assume???) but I'm not sure how to pass a void pointer to the C++ init function from perl
Any pointers (no pun intended) would be appreciated

Best,
Mike

Replies are listed 'Best First'.
Re: interface perl to C++ code with SWIG, re: void pointers?
by syphilis (Archbishop) on Aug 26, 2017 at 09:15 UTC
    TypeError in method 'version', argument 1 of type 'void *' at ./test_swig.pl line 8

    You've specified in your C++ code that the version method takes a single argument, but when you call that method from perl you provide no argument at all.
    I don't really know what version() is supposed to do, but if the correct way to call it from perl is to provide no arguments, then the C++ code should also declare version() as taking no arguments: 
    int version();

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    or, if C++ complains about that, try: 
    int version(void);

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    UPDATE: Duh ... my stupidity ... $control->version() does, of course, pass one argument to the version function.

    Sorry - I have some experience with C and perl, but little experience with C++, and virtually no experience with SWIG.
    A more common approach to having perl access C++ code is via the perl module Inline::CPP (instead of SWIG).

    Cheers,
    Rob
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Re: interface perl to C++ code with SWIG, re: void pointers?
by syphilis (Archbishop) on Aug 27, 2017 at 00:56 UTC
    After my initial gaff, I'll have another crack at providing something that might be marginally useful - and I second stevieb's request for more info.
    Inline::CPP and (Inline::C) seem to be quite happy to directly access the library's version() function: Here's the Inline::CPP demo: 
    ################
package FOO;
use strict;
use warnings;

use Inline CPP => Config =>
 BUILD_NOISY => 1, 
 ;

use Inline CPP => <<'EOC';

int version(void * p) {
  return 42;
}

EOC

my $sv = 'whatever';
my $obj = bless \$sv, 'FOO';
print $obj->version();
################

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    That works just fine and outputs '42'.
    Of course, it's a little bit different in your case because version() is in a separate library, so your script would be something like: 
    ################
package FOO;
use strict;
use warnings;

use Inline CPP => Config =>
 INC => '/path/to/include',
 LIBS => '-L/path/to/library -lmy_lib',
 BUILD_NOISY => 1,
 ;

use Inline CPP => <<'EOC';

#include <my_lib.h>

int wrap_version(SV * p) {
  return version(p);
}

EOC

my $sv = 'whatever';
my $obj = bless \$sv, 'FOO';
print $obj->wrap_version();
################

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    or, if you want to call the sub from perl as "version": 
    ################
package FOO;
use strict;
use warnings;

use Inline CPP => Config =>
 PREFIX => 'wrap_',
 INC => '/path/to/include',
 LIBS => '-L/path/to/library -lmy_lib',
 BUILD_NOISY => 1,
 ;

use Inline CPP => <<'EOC';

#include <my_lib.h>

int wrap_version(SV * p) {
  return version(p);
}

EOC

my $sv = 'whatever';
my $obj = bless \$sv, 'FOO';
print $obj->version();
################

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    I think SWIG should be capable of providing the same result.

    Cheers,
    Rob
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Re: interface perl to C++ code with SWIG, re: void pointers?
by stevieb (Canon) on Aug 26, 2017 at 21:39 UTC

    Could you please post a minimalistic base of your C++ code that allows for the instantiation of an object, and at least one of its methods?

    That will allow us to prototype/test the code in different scenarios and get you an example working result.

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