Provide typemaps for boost or std::optional<T>

[nanan14]

Hi ,

is there a way to typemap boost::optional to c# int? and vice versa
and also for other type like bool , string and other classes?

so far I only found this implementation.
https://stackoverflow.com/questions/15912937/can-you-swig-a-boostoptional

%template(optionalInt) optional<int>;
%typemap(ctype) optional<int> "void *"; 
%typemap(imtype) optional<int> "global::System.IntPtr"; 
%typemap(cstype) optional<int> "int?"; 
%typemap(csin) optional<int> "$csinput";
%typemap(out) optional<int> %{
    std::vector<int> result_vec;
    if (!!$1)
    {
        result_vec = std::vector<int>(0, $1.get());
    }
    else
    {
        result_vec = std::vector<int>();
    }
    $result = new std::vector< int >((const std::vector< int > &)result_vec); 
%}
%typemap(csout, excode=SWIGEXCODE) boost::optional<int> {
     global::System.IntPtr cPtr =$imcall;$excode
	 if (cPtr != global::System.IntPtr.Zero)
	 {
	     return (int)($imcall);
	 }
	 return null;
}
%typemap(csvarout, excode=SWIGEXCODE) optional<int>
%{ 
    get 
    { 
         global::System.IntPtr cPtr =$imcall;$excode
	    if (cPtr != global::System.IntPtr.Zero)
	    {
	        return (int)($imcall);
	    }
	    return null;
    } 
%}

[vadz]

There are no standard typemaps for this in SWIG but I do have my own typemaps for std::optional-like class for C#, Java and Python (OK, so the latter one is trivial, but still). I could tidy them up for the inclusion in SWIG itself if there is sufficient interest.

[aksswami]

It would be great if you can share the snippet as I am also looking for the same. I am new to the SWIG and type mapping complex type is something I don’t understand as of now.

[wsfulton]

@vadz, yes! They ought to port to std::optional which we ought to also have in SWIG.

[nanan14]

@vadz yes can you guide me in the right track.

[diegoferigo]

@vadz Is the std::optional support a feature you are still working on? Can you please provide the snippet you mentioned in the meantime?

[vadz]

No, sorry, other higher priority tasks took over and I was never able to work on this.

I've created some gists showing the typemaps we're using:

1. Main file including language-specific typemaps definitions.
2. C# typemaps (you may not need VBA-specific stuff)
3. Java typemaps (requires Java 8)
4. Python typemaps

These typemaps are for a very simple custom OptionalValue<> type, but should be straightforward to adapt for std::optional<>.

[diegoferigo]

Thanks a lot @vadz, your files have been very helpful. I am currently using a very simple swig configuration for std::optional using this optional.i. It requires to instantiate templates with something similar to the following:

%include "optional.i"
%template(Optional_i) std::optional<int>;

In your example I have't understood the logic behind DEFINE_OPTIONAL_CLASS and DEFINE_OPTIONAL_SIMPLE. Do you have to instantiate your templates with %template?

---

I tried to make something more complex, similar to a templated typemap in order to have a more idiomatic usage from python, but I couldn't make it work. For instance, I would like to directly map optional numeric types and strings to the target language. In python:

# Using the C++ signature
# std::optional<int> constructAndReturnOptional(const std::optional<int>&)

myint = mymodule.constructAndReturnOptional(42)
type(myint) # <class 'int'>

myint = mymodule.constructAndReturnOptional()
type(myint) # <class 'NoneType'>

I am pretty sure that it can be done using templated fragments together with $typemap(out,T) in language-specific typemaps, but I don't have enough experience with swig.

Does anyone have any example? I tried to understand how is done for std::vector but I didn't get it entirely.

[vadz]

In your example I have't understood the logic behind DEFINE_OPTIONAL_CLASS and DEFINE_OPTIONAL_SIMPLE.

In C# the representation of C++ optional<T> is either T? if it's a simple class or just T if it's a class. A single macro wouldn't be able to distinguish between these 2 cases (or, at least, I don't know how would it do it), so I've created 2 of them.

Do you have to instantiate your templates with %template?

No, the macros contain it.

For idiomatic usage, you really shouldn't be using Optional_i at all. Just use integers or None on the Python side.

[diegoferigo]

In C# the representation of C++ optional is either T? if it's a simple class or just T if it's a class. A single macro wouldn't be able to distinguish between these 2 cases (or, at least, I don't know how would it do it), so I've created 2 of them.

I'm not really familiar with C#, but I think I got your point.

For idiomatic usage, you really shouldn't be using Optional_i at all. Just use integers or None on the Python side.

I updated my comment above to reflect what I really meant. Now it should be more clear.

[JacobVarghese1992]

Hey @vadz , Sorry for a noob question. I'm looking at your java version. What exactly is OptionalValue in there. I get a Template 'OptionalValue' is undefined error. I just replaced it with optional and std::optional and get the same error, just with optional.

EDIT: That's your custom optional class nm.

[vadz]

OptionalValue is our custom type similar to boost::optional and std::optional, you should replace it with the type you actually use for your optional values.

[SmrtSmrtnik]

+1 for feature request, for what it's worth.
i have lots of c++ code with std::optional all over the place I've been tasked to SWIGify into Java, so definitely going to be looking at @vadz link and https://stackoverflow.com/questions/30272562/swig-wrap-boostoptional-in-java + the JNI spec as I've never done JNI before (and new to SWIG). Good times.

[lluisalemanypuig]

+1 for feature request. I am also very interested in seeing this implemented in SWIG.