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.

