++ed by:
ILUX AERO

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Author image 🦖🦜Graham Ollis🦃🦕

NAME

FFI::Platypus::Lang::CPP - Documentation and tools for using Platypus with the C++ programming language

SYNOPSIS

NOTE: The original author of this module considered the techniques used by and documented by this module to be somewhate experimental even back in 2015 when he wrote it. The original author now thinks that it is probably safer to write a C API layer between your C++ library and Perl rather than try to call C++ directly as advocated by this module. While the original author has not yet deprecated this module, users of this module should consider its limitations before using it.

C++:

 // on Linux compile with: g++ --shared -o basic.so basic.cpp
 // elsewhere, consult your C++ compiler documentation
 
 class Foo {
 
 public:
 
   // note you should avoid inlining functions
   // for classes you intend to use with FFI
   // as the compiler may not emit code/symbols
   // for those functions.
   Foo();
   ~Foo();
 
   int get_bar();
   void set_bar(int);
 
   int _size();
 
 private:
 
   int bar;
 
 };
 
 Foo::Foo()
 {
   bar = 0;
 }
 
 Foo::~Foo()
 {
 }
 
 int
 Foo::get_bar()
 {
   return bar;
 }
 
 void
 Foo::set_bar(int value)
 {
   bar = value;
 }
 
 int
 Foo::_size()
 {
   return sizeof(Foo);
 }

Perl:

 package Foo;
 
 use FFI::Platypus 1.00;
 use FFI::Platypus::Memory qw( malloc free );
 
 my $ffi = FFI::Platypus->new( api => 1 )
 $ffi->lang('CPP');
 $ffi->lib('./basic.so');
 
 $ffi->custom_type( Foo => {
   native_type => 'opaque',
   perl_to_native => sub { ${ $_[0] } },
   native_to_perl => sub { bless \$_[0], 'Foo' },
 });
 
 $ffi->attach( [ 'Foo::Foo()'     => '_new'     ] => ['Foo']  => 'void' );
 $ffi->attach( [ 'Foo::~Foo()'    => '_DESTROY' ] => ['Foo']  => 'void' );
 $ffi->attach( [ 'Foo::get_bar()' => 'get_bar'  ] => ['Foo']  => 'int'  );
 $ffi->attach( [ 'Foo::set_bar(int)'
                                  => 'set_bar'  ] => ['Foo','int']
                                                              => 'void' );
 
 my $size = $ffi->function('Foo::_size()' => [] => 'int')->call;
 
 sub new
 {
   my($class) = @_;
   my $ptr = malloc $size;
   my $self = bless \$ptr, $class;
   _new($self);
   $self;
 }
 
 sub DESTROY
 {
   my($self) = @_;
   _DESTROY($self);
   free($$self);
 }
 
 package main;
 
 my $foo = Foo->new;
 
 print $foo->get_bar, "\n";  # 0
 $foo->set_bar(22);
 print $foo->get_bar. "\n";  # 22

DESCRIPTION

This module provides some hooks for Platypus so that C++ names can be mangled for you. It uses the same primitive types as C. This document also documents issues and caveats that I have discovered in my attempts to work with C++ and FFI.

This module is somewhat experimental. It is also available for adoption for anyone either sufficiently knowledgable about C++ or eager enough to learn enough about C++. If you are interested, please send me a pull request or two on the project's GitHub.

There are numerous difficulties and caveats involved in using C++ libraries from Perl via FFI. This document is intended to enlighten on that subject.

Note that in addition to using pre-compiled C++ libraries you can bundle C++ code with your Perl distribution using FFI::Build. For a complete example, which attempts to address the caveats listed below you can take a look at this sample distro on GitHub:

https://github.com/plicease/Color-FFI

CAVEATS

In general I have done my research of FFI and C++ using the Gnu C++ compiler. I have done some testing with clang as well.

name mangling

C++ names are "mangled" to handle features such as function overloading and the fact that some characters in the C++ names are illegal machine code symbol names. What this means is that the C++ member function Foo::get_bar looks like _ZN3Foo7get_barEv to FFI::Platypus. What makes this even trickier is that different C++ compilers provide different mangling formats. When you use the FFI::Platypus#lang method to tell Platypus that you are intending to use it with C++, like this:

 $ffi->lang('CPP');

it will mangle the names that you give it. That saves you having to figure out the "real" name for Foo::get_bar.

The current implementation uses the c++filt command or FFI::Platypus::Lang::CPP::Demangle::XS if it is installed. If c++filt cannot be found at install time, then FFI::Platypus::Lang::CPP::Demangle::XS will be made a prerequsite, so you can have some confidence that this feature will work even if your platform does not provide c++filt. The XS module is not a prerequsite when c++filt IS found because using c++filt does not require invoking the compiler and may be more reliable.

If the approach to mangling C++ names described above does not work for you, or if it makes you feel slightly queasy, then you can also write C wrapper functions around each C++ method that you want to call from Perl. You can write these wrapper functions right in your C++ code using the extern "C" trick:

 class Foo {
   public:
     int bar() { return 1; }
 }
 
 extern "C" int
 my_bar(Class *foo)
 {
   return foo->bar();
 }

Then instead of attaching Foo::bar() attach my_bar.

 $ffi->attach( my_bar => [ 'Foo' ] => 'int' );

constructors, destructors and methods

Constructors and destructors are essentially just functions that do not return a value that need to be called when the object is created and when it is no longer needed (respectively). They take a pointer to the object (this) as their first argument. Constructors can take additional arguments, as you might expect they just come after the object itself. Destructors take no arguments other than the object itself (this).

You need to alloate the memory needed for the object before you call the constructor and free it after calling the destructor. The tricky bit is figuring out how much memory to allocate. If you have access to the header file that describes the class and a compiler you can compute the size from within C++ and hand it off to Perl using a static method as I did in the "SYNOPSIS" above.

Regular methods also take the object pointer as their first argument. Additional arguments follow, and they may or may not return a value.

inline functions

C++ compilers typically do not emit symbols for inlined functions. If you get a message like this:

 unable to find Foo::get_bar() at basic line 21

even though you are sure that class has that method, this is probably the problem that you are having. The Gnu C++ compiler, g++ has an option to force it to emit the symbols, even for inlined functions:

 -fkeep-inline-functions     # use this

Clang has an option to do the opposite of this:

 -fvisibility-inlines-hidden # do not use this

but unhelpfully not a way to keep inlined functions. This appears to be a deliberate design decision made by the clang developers and it makes sense for C++, since inline functions are typically defined in C++ header files (.h) so it is difficult to determine in which object file the uninlined inlined functions should go.

If you have the source of the C++ and you can recompile it you can also optionally change it to not use inlined functions. In addition to removing any inline keywords from the source, you need to move the implementations of any methods outside of the class body. That is, do not do this:

 class Foo {
   public:
     int bar() { return 1; } # WRONG
 }

Do this:

 class Foo {
   public:
     int bar();              # RIGHT
 }
 
 int
 Foo::bar()                  # RIGHT
 {
   return 1;
 }

the standard C++ library

If you are getting errors like this:

 unable to find Foo::Foo()

that can't be explained by the issues described above, set the environment variable FFI_PLATYPUS_DLERROR to a true value and try again. If you see a warning like this:

 error loading Foo.so: Foo.so: undefined symbol: __gxx_personality_v0

then you probably need to explicitly link with the standard C++ library. The most portable way to deal with this is by using ExtUtils::CppGuess.

METHODS

Generally you will not use this class directly, instead interacting with the FFI::Platypus instance. However, the public methods used by Platypus are documented here.

native_type_map

 my $hashref = FFI::Platypus::Lang::CPP->native_type_map;

This returns a hash reference containing the native aliases for the C++ programming languages. That is the keys are native C++ types and the values are libffi native types.

mangler

 my $mangler = FFI::Platypus::Lang::CPP->mangler($ffi->libs);
 # prints _ZN9MyInteger7int_sumEii
 print $mangler->("MyInteger::int_sum(int, int)");

Returns a subroutine reference that will "mangle" C++ names.

EXAMPLES

Using a C++ class without writing bundling any C/C++ code

The example in the "SYNOPSIS" shows how you can use a C++ class without writing any wrapper code, though you will have to guess or determine the instance size of the class.

Using a C++ class with a wrapper

(For the full source for this example, see examples/wrapper.{pl,cpp} that came with this distribution)

Sometimes it is easier to write wrapper functions around your new and delete operations. Consider if you add these functions to the C++ source to the example in the "SYNOPSIS".

 // These could also be class methods
 extern "C" Foo*
 Foo_new()
 {
   return new Foo();
 }
 
 extern "C" void
 Foo_delete(Foo *foo)
 {
   delete foo;
 }

Now we can use this class without having to know in the perl code what the size of the class is. We declare the constructor and destructor in Perl space like this:

 $ffi->attach( [ 'Foo_new'        => 'new'      ] => []       => 'Foo' );
 $ffi->attach( [ 'Foo_delete'     => 'DESTROY'  ] => ['Foo']  => 'void' );

We've also removed the Perl new and DESTROY wrappers as they are unnecessary now, and so the the C++ functions are attached directly to their intended names.

Exceptions

(For the full source of this example, see examples/exception.{pl,cpp} that came with this distribution)

If your library throws an exception and you do not catch it in C++ it is going to kill your program. As an example, suppose set_bar in the example above throws an exception:

 void
 Foo::set_bar(int value)
 {
   if(value > 512)
     throw new FooException("too hot");
   if(value < 0)
     throw new FooException("too cold");
    bar = value;
 }

Now if you try to use set_bar with a bad value like this from Perl:

 $foo->set_bar(-2);

it will crash your Perl program.

 terminate called after throwing an instance of 'FooException'
 Abort

To handle this, you need to write a wrapper around the set_bar method.

 static FooException *last_exception = NULL;
 
 extern "C" FooException *
 Foo_get_exception()
 {
   return last_exception;
 }
 
 extern "C" void
 Foo_reset_exception()
 {
   if(last_exception != NULL)
     delete last_exception;
   last_exception = NULL;
 }
 
 extern "C" void
 Foo_set_bar(Foo *foo, int value)
 {
   try
   {
     Foo_reset_exception();
     foo->set_bar(value);
   }
   catch(FooException *e)
   {
     last_exception = e;
   }
 }

Next we will write an interface to the FooException class in Perl:

 package FooException;
 
 use overload '""' => sub { "exception: " . $_[0]->message . "\n" };
 
 $ffi->custom_type( FooException => {
   native_type => 'opaque',
   perl_to_native => sub { ${ $_[0] } },
   native_to_perl => sub {
     defined $_[0]
     ? (bless \$_[0], 'FooException')
     : ();
   },
 });
 
 $ffi->attach(
   [ 'Foo_get_exception' => 'get_exception' ] => [] => 'FooException'
 );
 
 $ffi->attach(
   [ 'FooException::message()' => 'message' ] => ['FooException'] => 'string'
 );

And finally we write a wrapper for the Perl set_bar method.

 $ffi->attach( [ 'Foo_set_bar'    => '_set_bar' ] => ['Foo','int']
                                                              => 'void' );
 sub set_bar
 {
   my($self, $value) = @_;
   $self->_set_bar($value);
   my $error = FooException->get_exception;
   die $error if $error;
 }

And now when we give set_bar a bogus value we get a Perl exception instead of an application crash:

 exception: too cold

So we can easily wrap the call to set_bar in a Perl eval if we want to catch the exception and handle it.

SUPPORT

If something does not work as advertised, or the way that you think it should, or if you have a feature request, please open an issue on this project's GitHub issue tracker:

https://github.com/PerlFFI/FFI-Platypus-Lang-CPP/issues

CONTRIBUTING

If you have implemented a new feature or fixed a bug then you may make a pull reequest on this project's GitHub repository:

https://github.com/PerlFFI/FFI-Platypus-Lang-CPP/issues

Caution: if you do this too frequently I may nominate you as the new maintainer. Extreme caution: if you like that sort of thing.

This project's GitHub issue tracker listed above is not Write-Only. If you want to contribute then feel free to browse through the existing issues and see if there is something you feel you might be good at and take a whack at the problem. I frequently open issues myself that I hope will be accomplished by someone in the future but do not have time to immediately implement myself.

Another good area to help out in is documentation. I try to make sure that there is good document coverage, that is there should be documentation describing all the public features and warnings about common pitfalls, but an outsider's or alternate view point on such things would be welcome; if you see something confusing or lacks sufficient detail I encourage documentation only pull requests to improve things.

SEE ALSO

FFI::Platypus

The Core Platypus documentation.

FFI::Build + FFI::Build::File::CXX

Bundle C or C++ with your FFI / Perl extension.

ExtUtils::CppGuess

Guess the appropriate C++ compiler / linker flags for your C compiler platform combination.

AUTHOR

Graham Ollis <plicease@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2015 by Graham Ollis.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.