C++ / const class pointer signature / unable to find correct D syntax

[Rubn]

On Friday, 4 May 2018 at 07:57:26 UTC, Uknown wrote:
> On Friday, 4 May 2018 at 07:49:02 UTC, Robert M. Münch wrote:
>> I have a static C++ and can't make it to get a correct binding 
>> for one function:
>>
>> DMD: public: unsigned int __cdecl b2d::Context2D::_begin(class 
>> b2d::Image & __ptr64,class b2d::Context2D::InitParams const * 
>> __ptr64 const) __ptr64
>> LIB: public: unsigned int __cdecl b2d::Context2D::_begin(class 
>> b2d::Image & __ptr64,class b2d::Context2D::InitParams const * 
>> __ptr64) __ptr64
>>
>> So I somehow get some more const from D. This is the code I 
>> used:
>>
>>    final uint _begin(ref Image image, const(InitParams) 
>> initParams);
>>
>> Which creates a const pointer to a const class signature. But 
>> it should only be a const pointer. Any idea how to solve this?
>
> The problem is that const in D is transitive. That means T * 
> const from C++ is not expressible in D. Any reference through a 
> const becomes const. To use it, IMO your best bet is to make a 
> wrapper function on the C++ side like this:
>
> unsigned int __cdecl b2d::Context2D::_begin(class b2d::Image & 
> im,class b2d::Context2D::InitParams const * const InitParams)
> {
>     return //the call to the original c++ function
> }
>
> Alternatively you can use dpp, or dstep or some similar tool to 
> try and let the tool create bindings. As a last ditch, you can 
> force the mangle to match by using pragma(mangle, ...) like 
> this:
>
> pragma(mangle, _ZactualMangleFromC++Compiler)
> final uint _begin(ref Image image, const(InitParams) 
> initParams);

Using pragma(mangle) isn't really a solution though, unless you 
only plan on using one platform/compiler. C++ doesn't have a 
single standard name mangling. You would need to get the name 
mangling for every compiler you plan to use then use a version 
define fore each. Which isn't really practical for how many 
bindings you would need to do that for.

The easiest solution is to just write C bindings for the C++ 
library then use those instead from D.