Re: [Swig-user] Swig C# NullException problem

[William S F. <ws...@fu...>]

On 18 June 2015 at 19:45, Prof. Christoph Müller <mc...@hs...> wrote:
>> Date: Wed, 10 Jun 2015 22:38:59 +0100
>> From: William S Fulton <ws...@fu...>
>> Subject: Re: [Swig-user] Swig C# NullException problem
>> To: Linda Schey <lin...@gm...>
>> Cc: swig-user <swi...@li...>
>> Message-ID:
>>       <CAN...@ma...>
>> Content-Type: text/plain; charset=UTF-8
>>
>> On 8 June 2015 at 16:12, Linda Schey <lin...@gm...> wrote:
>> > Hello,
>> >
>> > we have encountered a problem with swig for which we couln't find any
>> > solution in the documentation nor in forum posts.
>> > We are right now developing an universal plugin which will connect the
>> > 3d program Blender with the 3d engine of our university. Blenders API
>> > is written in c/c++ whereas our engine is developed in c#.
>> > We need to swig a lot of classes which lead us to write typemaps that
>> > won't address individual cases but set up rules for all custom types
> etc.
>> >
>> > It's a huge learning process for us but right now we are having
>> > troubles with the exception handling or at least it seems like this is
>> > the error source.
>> > Swig will always create a default exception in the generated c++ file
>> > which results in zero as a return value. But this will create a
>> > compile error with the method of our custom class, that can't have zero
> as a return value.
>> >
>> > We've already gone through the documentation chapters about exception
>> > handling and couldn't find a solution for our problem.
>> >
>> > How can we set up a custom exception that will result in a default
>> > value if the object is null? Is there an obvious mistake we're just
>> > too narrow minded to see? Can the error be fixed in another way? (The
>> > object can never be null which means we really don't need this
>> > exception.)
>> >
>> > Please find below our sample code which includes our typemaps (for the
>> > exception handling and other custom types) and the generated c++ file.
>> >
>> > A huge thank you in advance for any clues and with best wishes,
>> >
>> > Linda Schey and Sarah Haefele
>> > Computer Science in Media Master, Furtwangen University Germany
>> >
>> > --------------------------------------
>> > cpp_file.h
>> > --------------------------------------
>> > std::array<float, 3> FooInArrayOut(Foo f);
>> >
>> > --------------------------------------
>> > cpp_file.i
>> > --------------------------------------
>> > //Map std::array<float,3> To Fusee.Math.float3
>> > %typemap(ctype)  std::array<float, 3> "std::array<float, 3> /*
>> > std::array<float, 3>_ctype */"
>> > %typemap(imtype) std::array<float, 3> "Fusee.Math.float3 /*
>> > Fusee.Math.float3*_imtype */"
>> > %typemap(cstype) std::array<float, 3> "Fusee.Math.float3 /*
>> > iFusee.Math.float3_cstype */"
>> > %typemap(csin)   std::array<float, 3> "$csinput /*
> Fusee.Math.float3_csin
>> > */"
>> > %typemap(in)     std::array<float, 3> %{ $1 = $input /*
> Fusee.Math.float3
>> > in*/; %}
>> > %typemap(out)    std::array<float, 3> %{ /* Fusee.Math.float3
> out*/$result =
>> > $1/* Fusee.Math.float3 out*/; %}
>> > %typemap(csout)  std::array<float, 3> { return $imcall/*
>> > Fusee.Math.float3 csout*/; }
>> >
>> > --------------------------------------
>> > cpp_file_wrap.cxx
>> > --------------------------------------
>> >  SWIGEXPORT std::array<float, 3> /* std::array<float, 3>_ctype */
>> > SWIGSTDCALL CSharp_cpp_file_FooInArrayOut(void * jarg1, void * jarg2) {
>> >   std::array<float, 3> /* std::array<float, 3>_ctype */ jresult ;
>> >   cpp_file *arg1 = (cpp_file *) 0 ;
>> >   Foo arg2 ;
>> >   Foo *argp2 ;
>> >   std::array< float,3 > result;
>> >
>> >   arg1 = (cpp_file *)jarg1;
>> >   argp2 = (Foo *)jarg2;
>> >   if (!argp2) {
>> >
>> > SWIG_CSharpSetPendingExceptionArgument(SWIG_CSharpArgumentNullExceptio
>> > n, "Attempt to dereference null Foo", 0);
>> >     return 0;
>> >   }
>> >   arg2 = *argp2;
>> >   result = (arg1)->FooInArrayOut(arg2);
>> >   /* Fusee.Math.float3 out*/jresult = result/* Fusee.Math.float3 out*/;
>> >   return jresult;
>> > }
>> >
>> > --------------------------------------
>> > Error Msg
>> > --------------------------------------
>> > error C2440: 'return' : cannot convert from 'int' to
> 'std::array<float,3>'
>>
>>
>> I don't think you can use std::array<float, 3> as a return type in wrapper
> function. How is the CLI going to know how to marshal this type to
> Fusee.Math.float3 ?
>>
>> The exception handling is written such that the return type return type
> will be void or a type that will accept zero. I don't think any other type
> will easily be handling without overriding quite a bit of default code
> generation.
>>
>> William
>>
>
> The CLI is capable of marshalling any C/C++ POD (plain-old-datatype -
> classes or structs without methods, con- or destructors) to a respective C#
> struct with the same memory layout. We are using this scheme to wrap c++
> methods returning instances (not pointers) to c# methods returning structs.
> It works very well in a big project where we SWIGged the C++ API of Cinema4D
> (currently more than 700 classes wrapped - http://fusee3d.org/c4dexporter/)
> and we're very happy with it. The funny thing is: This only works because
> just by chance none of the numerous C++methods returning a POD on the stack
> takes a pointer to a class as an argument:
>
> A method like this works with the approach mentioned by Linda:
> Vector TestReturnStructOnStack(); // (WORKS JUST FINE)
>
> And a method like that doesn't - because the SWIG-generated C++-Stub tries
> to return 0 (which is not castable to Vector):
> Vector TestReturnStructOnStack(Foo *foo); // (C++ COMPILE ERROR in
> SWIG-C++-Stub - cannot cast int to Vector)
>
> I doubt that this is by design. There should be a customizable typemap-entry
> for "Vector" allowing to change the default return value of any method
> returning "Vector".
> I tried to add a constructor taking an int to Vector. This cures the
> compiler error but then Vector is not a POD anymore and the p/invoke will
> crash.
> Thanks for any help - we have been struggling with this for weeks now.

Ah yes of course, marshalling of value types and std::array can be a
POD. There is actually a solution. You'll notice that the SWIG
exception code uses ' return $null' where $null is a special variable.
SWIG expands $null into 0 unless the 'out' typemap has a 'null'
attribute, in which case it uses the value of this attribute. This is
used for void returns where the 'out' typemap in Lib/csharp/csharp.swg
is:
  %typemap(out, null="") void ""
so 'return' is generated instead of 'return 0'.

So for your case, just modify the out typemap and add in the 'null'
attribute as follows:

%typemap(out, null="std::array<float, 3>()")    std::array<float, 3>
%{ /* Fusee.Math.float3 out*/$result = $1/* Fusee.Math.float3 out*/;
%}

The SWIG documentation ought to have some good examples of value types
typemaps such as the std::array you show and it also ought to have
this explained. A patch to the docs in Doc/Manual/CSharp.html would be
much appreciated!

William