Calling C++ functions from Perl without using xsubpp

[dafrito]

I'm writing a Perl-to-C++ bridge, and I've ran into a roadblock regarding calling arbitrary C++ functions from Perl. As I understand it, XS would be used to compile headers to allow invocation from Perl. However, I would like this bridge to work without the need to compile separate headers. Let me explain firstly why I don't think this is crazy:

When Perl invokes a C function, it provides a stack of arguments. Through the use of C++ tuples and function overloading, I would be able to convert each Perl argument to the correct C++ type. Once all arguments have been converted, I would then invoke the target C++ function.

So, my question is: How do I pass a C++ function into Perl? Specifically, what function should be used to pass my function pointer into perl, and what should the signature of that function look like?

I'll start looking into code generated by xsubpp, but I'd love some pointing in the right direction if you guys have any.

Thanks in advance!

[davido]

Inline::CPP does pretty much what you're asking. It doesn't completely absolve you from studying perlguts, but it's a big step in that direction.

use Inline CPP => 'DATA';

my $sum = add( 2, 4 );
print "$sum\n";

__DATA__
__CPP__

#include <iostream>

int add( int x, int y ) {
    return x + y;
}

There are C++ constructs that don't map well to Perl. For one thing, your C++ compiler has no idea how Perl intends to call a function, so template-based functions and classes need to be instantiated in a concrete C++ function or class wrapper before being exposed to Perl. But once you get familiar with the large subset of C++ that does map well to Perl, Inline::CPP is pretty fun to use. I've even spent some time verifying its compatibility with the new C++11 standard (much of it "just works" as it should, with a few exceptions in predictable places).

The Inline::CPP documentation (always a work in progress) is improving, and should get you going in the right direction. I also recommend reading Inline, Inline::C, and Inline::C-Cookbook. Lots of reading, I know. But each of those resources gets you closer to being productive in this funky art. The Inline::C-Cookbook, in particular, demonstrates how to manipulate the Perl stack within your C or C++ functions.

Also, the tests found in the Inline::CPP distribution under t/ and grammar/t demonstrate many C++ constructs at work. And Math::Prime::FastSieve is a sort of "proof of concept" and example of using Inline::CPP.

Advanced Perl Programming (2nd Edition) has a chapter that discusses the Inline modules (Inline::C in particular, but much is applicable to Inline::CPP). And Sam Treagar's book, "Writing Perl Modules for CPAN" has a chapter on Inline::C too.

If you find yourself working much with it, feel free to join the [e-mail] mailing list. See http://lists.perl.org for details.

[Corion]

Have you looked at Inline::CPP, which allows you to seamlessly call C++ code from Perl and likely also Perl code from C++?

[Anonymous Monk]

:) Inline::CPP, http://swig.org/exec.html, ExtUtils::XSpp

site:perlmonks.org Inline::CPP swig XSpp -> Interfacing Perl with C++, using XS, with external files, and using the STL as parameters and return values.

[andal]

Actually, xsubpp produces set of code that does conversion from structures specific to perl to those understandable for your code. In fact, you can write C code that works with perl structures directly, in which case no conversion has to be done. Just make your function work with AV* or for example SV*.

Inline::C and Co. provide automatic conversion to some set of common types, just like XS does. If using these types is enough, then you don't need anything else but those modules. If you have to work with more complex data structures, then you have to understand perl's guts. Read perldoc perlguts. Then read perldoc perlxs to learn how to work with XS. Then read perldoc perlapi to see which functions are available to you for working with perl's structures. XS shall only simplify certain things for you.

After all, you can think of Perl as of C library providing certain API.

[syphilis]

Inline::C and Co. provide automatic conversion to some set of common types, just like XS does. If using these types is enough, then you don't need anything else but those modules. If you have to work with more complex data structures, then you have to .... learn how to work with XS.

Perhaps I've misread (and inappropriately edited) that excerpt from your post, but it seems to me that you're saying that Inline::C is good only for some limited set of C types.
That's not the case - if it can be done using XS then it can be done using Inline::C. After all, Inline::C is little more than a utility that generates an XS file, which it then compiles.
(I keep thinking there must be *some* feature of XS that Inline::C can't reproduce ... and I'd love to see some examples that validate such thinking.)

Cheers,
Rob

[andal]

Well, I guess I have expressed myself imprecise. My point was not so much about abilities of Inline::C. My point was about need to learn perl guts if one needs to work with more complex input/output parameters. In this case learning XS comes in mostly for understanding what is happening. Though, probably some people can write code without knowing how their code turns into program :)