Re: Using std_string.i to automatically convert a different C++ string class to ruby String

I would run swig with the -E option using this input interface file:

  %module example
  %include <std_string.i>

This will output the preprocessed code with the std::string typemaps at the
end of the output. I advise this as the input files are full of macros and
may be hard to follow. I'd copy the std::string typemaps and tweak for
QString. The typemaps for std::string are comprehensive, you may only need
a get away with just the in, out, typecheck typemaps for simple usage.

Typemaps for QT types ought to go into a specific QT SWIG specific
repository, please take a look at https://github.com/swig/swig/issues/88.

William

On Tue, 11 Apr 2023 at 12:55, Stefano Crocco <[email protected]>
wrote:

> Hello to everyone,
> I'm new to SWIG and I'm trying to use SWIG to use the Qt C++ libraries [1]
> from Ruby. After some simple tests, I tried to wrap some functions using
> strings. Unfortunately, Qt doesn't use std::string to represent strings,
> but
> its own class, called QString. I'd like to be able to automatically
> convert
> QString to and from ruby String, just as the typemaps in std_string.i does
> for
> std::string.
>
> I hoped that, since QString can be converted to std::string and vice
> versa, it
> would be possible to somehow make use of the std_string.i library to tell
> SWIG:
> - when, converting from C++ to ruby, you find a QString, call the
> QString::toStdString method to convert it to std::string, then apply the
> default typemap for std::string
> - when, converting from ruby to C++, you find String, convert it to a
> std::string using the default typemap for std::string, then call
> QString::fromStdString on the result.
>
> Unfortunately, as far as I can tell, this isn't possible. I hoped that
> typemaps would be applied recursively, so I tried the following:
>
> %typemap(out) QString {
>   $result = $1.toStdString();
> }
>
> I hoped that SWIG would then notice that the resulting value wasn't a ruby
> object but it still was a C++ class it knows how to convert to ruby. Of
> course, that didn't work and I got the error:
> error: cannot convert ‘std::string’ {aka
> ‘std::__cxx11::basic_string<char>’}
> to ‘VALUE’ {aka ‘long unsigned int’} in assignment
>
> Is there some way to converting QString to and from ruby String using,
> directly or indirectly, the existing conversions of std::string? I'd like
> to
> avoid having to avoid writing the low-level conversion code myself, unless
> absolutely necessary. I searched both the documentation and Google and
> looked
> at the code in std_string.i, but I couldn't find anything useful, so I
> guess
> it's not possible, but I'd like to be sure.
>
> Thanks in advance
>
> Stefano
>
>
> [1] https://www.qt.io/
>
>
>
>
> _______________________________________________
> Swig-user mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/swig-user
>

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