C/C++ struct interfacing

["Rémy Mouëza"]

Below is a solution to get it to work as a C binding.

- Your structure should not be enclosed in the `extern (C)` 
block; leave it as regular D code.
- cppcall should be declared `extern (C)` in D and `extern "C"` 
in C++;
- depending on your platform, you need to link to the C++ 
standard lib ; on my Ubuntu 12.04, I added the `-L-lstdc++` on 
the dmd command line.


I already used that pattern in some binding for the RtMidi lib:
https://github.com/remy-j-a-moueza/drtmidi

dRtMidi.d defines a templated structure :

struct answer (T) {
     int success;
     T value;
     const (char) * errMsg;
}

and so does cRtMidi.cpp :

template <typename T>
struct answer {
     int success;
     T value;
     const char * errMsg;
};

I declared a bunch of `extern (C)` and `extern "C"` function that 
pass that data arround. It works only if the T data type as the 
same size in both languages.

You can check the size correspondance on this page : 
http://dlang.org/interfaceToC.html

And for more tricky types, you can get some inspiration from 
Jacob Carlborg dstep project, especially :
- translateType () in Type.d : 
https://github.com/jacob-carlborg/dstep/blob/master/dstep/translator/Type.d

- and the static constructor of the IncludeHandler class, to get 
to import the right D core module : 
https://github.com/jacob-carlborg/dstep/blob/master/dstep/translator/IncludeHandler.d




On Sunday, 27 October 2013 at 12:20:49 UTC, Oleg B wrote:
> Hello.
> I want use one struct in two language.
> How to declare the same structure in different languages​​?
>
> D code: calling C++ function with my struct
> <code>
> extern(C++)
> {
>     struct vec(size_t N, T=float)
>     { // << line 6
>         alias vec!(N,T) thistype;
>         T[N] data;
>
>         auto opBinary(string op)( in thistype rhs ) const
>         {
>             thistype ret;
>             foreach( i; 0 .. N ) ret.data[i] = data[i] + 
> rhs.data[i];
>             return ret;
>         }
>     }
>
>     void cppcall( vec!(3,float) d );
> }
>
> void main()
> {
>     vec!3 a, b;
>     a.data[0] = 10;
>     a.data[1] = 7;
>     a.data[2] = 3;
>
>     b.data[0] = 13;
>     b.data[1] = 9;
>     b.data[2] = 4;
>
>     cppcall( a + b );
> }
> </code>
>
> C++ code: use struct data
> <code>
> #include <cstdio>
>
> struct vec
> {
>     float data[3];
> };
>
> void cppcall( vec d )
> {
>     printf( "%f %f %f\n", d.data[0], d.data[1], d.data[2] );
> }
> </code>
>
> compilation failed with unclear error
>
> $ g++ -c ctypestest.cpp -o ctypestest.o
> $ dmd ctypestest.d ctypestest.o -ofctypestest
> ctypestest.d(6): Error: struct ctypestest.vec!(3, float).vec 
> C++ static variables not supported