The original C++ class
https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core/types.hpp#L315:
template<typename _Tp> class Size_
{
public:
    typedef _Tp value_type;

    //! default constructor
    Size_();
    Size_(_Tp _width, _Tp _height);
    Size_(const Size_& sz);
    Size_(Size_&& sz) CV_NOEXCEPT;
    Size_(const Point_<_Tp>& pt);

    Size_& operator = (const Size_& sz);
    Size_& operator = (Size_&& sz) CV_NOEXCEPT;
    //! the area (width*height)
    _Tp area() const;
    //! aspect ratio (width/height)
    double aspectRatio() const;
    //! true if empty
    bool empty() const;

    //! conversion of another data type.
    template<typename _Tp2> operator Size_<_Tp2>() const;

    _Tp width; //!< the width
    _Tp height; //!< the height
};

// my auxiliary cpp code:
cv::Size_<int>* createSizeIntWH(int _width, int _height){
    return new cv::Size_<int>(_width, _height);
}

void deleteSizeInt(cv::Size_<int> *&sz){
    delete sz;
}

// d code:
extern(C++, cv){
class Size_(_Tp){
    @disable this();

    final _Tp area() const;
    final double aspectRatio() const;
    final bool empty() const;

    _Tp width; //!< the width
    _Tp height; //!< the height
}
}

// my test code that fails:
Size_!int sz = createSizeIntWH(200, 100);
writeln(sz.width);

One of the problems is that sz.width is not printed as 200, but a random int.
Other problem is that if I try to call one of area, aspectRatio, and empty, it does not compile yielding a linker error:
error LNK2019: unresolved external symbol "public: int __cdecl cv::Size_<int>::area(void)const " (?area@?$Size_@H@cv@@QEBAHXZ) referenced in function _Dmain
Somehow linker cannot locate that symbol. Is this a name mangling issue?
In the same library I could successfully interface cv::Mat which is a template-free definition. I suspect if D allows interfacing C++ class templates since docs do not cover this, but only struct templates?

15.03.2020 22:39, Ferhat Kurtulmuş пишет:
> 
> The original C++ class
> https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core/types.hpp#L315: 
> 
> template<typename _Tp> class Size_
> {
> public:
>      typedef _Tp value_type;
> 
>      //! default constructor
>      Size_();
>      Size_(_Tp _width, _Tp _height);
>      Size_(const Size_& sz);
>      Size_(Size_&& sz) CV_NOEXCEPT;
>      Size_(const Point_<_Tp>& pt);
> 
>      Size_& operator = (const Size_& sz);
>      Size_& operator = (Size_&& sz) CV_NOEXCEPT;
>      //! the area (width*height)
>      _Tp area() const;
>      //! aspect ratio (width/height)
>      double aspectRatio() const;
>      //! true if empty
>      bool empty() const;
> 
>      //! conversion of another data type.
>      template<typename _Tp2> operator Size_<_Tp2>() const;
> 
>      _Tp width; //!< the width
>      _Tp height; //!< the height
> };
> 
> // my auxiliary cpp code:
> cv::Size_<int>* createSizeIntWH(int _width, int _height){
>      return new cv::Size_<int>(_width, _height);
> }
> 
> void deleteSizeInt(cv::Size_<int> *&sz){
>      delete sz;
> }
> 
> // d code:
> extern(C++, cv){
> class Size_(_Tp){
>      @disable this();
> 
>      final _Tp area() const;
>      final double aspectRatio() const;
>      final bool empty() const;
> 
>      _Tp width; //!< the width
>      _Tp height; //!< the height
> }
> }
> 
> // my test code that fails:
> Size_!int sz = createSizeIntWH(200, 100);
> writeln(sz.width);

What is the D version of `createSizeIntWH`? In C++ it returns a pointer but in D version it returns an instance.

> 
> One of the problems is that sz.width is not printed as 200, but a random int.
> Other problem is that if I try to call one of area, aspectRatio, and empty, it does not compile yielding a linker error:
> error LNK2019: unresolved external symbol "public: int __cdecl cv::Size_<int>::area(void)const " (?area@?$Size_@H@cv@@QEBAHXZ) referenced in function _Dmain
> Somehow linker cannot locate that symbol. Is this a name mangling issue?
> In the same library I could successfully interface cv::Mat which is a template-free definition. I suspect if D allows interfacing C++ class templates since docs do not cover this, but only struct templates?
> 

On Sunday, 15 March 2020 at 20:21:57 UTC, drug wrote:
> 15.03.2020 22:39, Ferhat Kurtulmuş пишет:
>> 

> What is the D version of `createSizeIntWH`? In C++ it returns a pointer but in D version it returns an instance.

extern(C++){
    cv.Size_!int createSizeInt();
    cv.Size_!int createSizeIntWH(int w, int h);
    void deleteSizeInt(ref cv.Size_!int sz);
}

15.03.2020 23:25, Ferhat Kurtulmuş пишет:
> On Sunday, 15 March 2020 at 20:21:57 UTC, drug wrote:
>> 15.03.2020 22:39, Ferhat Kurtulmuş пишет:
>>>
> 
>> What is the D version of `createSizeIntWH`? In C++ it returns a pointer but in D version it returns an instance.
> 
> extern(C++){
>      cv.Size_!int createSizeInt();
>      cv.Size_!int createSizeIntWH(int w, int h);
>      void deleteSizeInt(ref cv.Size_!int sz);
> }
> 

createSizeIntWH returns a pointer to instance, not an instance. Try:
cv.Size_!int* createSizeIntWH(int w, int h);

On Sunday, 15 March 2020 at 20:46:14 UTC, drug wrote:
> 15.03.2020 23:25, Ferhat Kurtulmuş пишет:
>> On Sunday, 15 March 2020 at 20:21:57 UTC, drug wrote:
>>> 15.03.2020 22:39, Ferhat Kurtulmuş пишет:
>>>>
>> 
>>> What is the D version of `createSizeIntWH`? In C++ it returns a pointer but in D version it returns an instance.
>> 
>> extern(C++){
>>      cv.Size_!int createSizeInt();
>>      cv.Size_!int createSizeIntWH(int w, int h);
>>      void deleteSizeInt(ref cv.Size_!int sz);
>> }
>> 
>
> createSizeIntWH returns a pointer to instance, not an instance. Try:
> cv.Size_!int* createSizeIntWH(int w, int h);

I doubt it because in https://dlang.org/spec/cpp_interface.html#using_cpp_classes_from_d

cpp code:
Derived *createInstance(int i)

d code:
extern (C++){
    ...
    Derived createInstance(int i);
}

15.03.2020 23:53, Ferhat Kurtulmuş пишет:
> 
> I doubt it because in https://dlang.org/spec/cpp_interface.html#using_cpp_classes_from_d
> 
> cpp code:
> Derived *createInstance(int i)
> 
> d code:
> extern (C++){
>      ...
>      Derived createInstance(int i);
> }
> 
> 
Ah, really, you use classes on D side.

On Sunday, 15 March 2020 at 20:53:49 UTC, Ferhat Kurtulmuş wrote:
> On Sunday, 15 March 2020 at 20:46:14 UTC, drug wrote:
>> 15.03.2020 23:25, Ferhat Kurtulmuş пишет:
>>> On Sunday, 15 March 2020 at 20:21:57 UTC, drug wrote:
>>>> 15.03.2020 22:39, Ferhat Kurtulmuş пишет:
>>>>>
>>> 
>>>> What is the D version of `createSizeIntWH`? In C++ it returns a pointer but in D version it returns an instance.
>>> 
>>> extern(C++){
>>>      cv.Size_!int createSizeInt();
>>>      cv.Size_!int createSizeIntWH(int w, int h);
>>>      void deleteSizeInt(ref cv.Size_!int sz);
>>> }
>>> 
>>
>> createSizeIntWH returns a pointer to instance, not an instance. Try:
>> cv.Size_!int* createSizeIntWH(int w, int h);
>
> I doubt it because in https://dlang.org/spec/cpp_interface.html#using_cpp_classes_from_d
>
> cpp code:
> Derived *createInstance(int i)
>
> d code:
> extern (C++){
>     ...
>     Derived createInstance(int i);
> }

I wouldn't use a class on the D side, unless your C++ type uses virtual functions. Classes in D are different from structs, it is not the same as C++ where they are basically the same thing.

On Sunday, 15 March 2020 at 21:16:43 UTC, Arine wrote:
> On Sunday, 15 March 2020 at 20:53:49 UTC, Ferhat Kurtulmuş

> I wouldn't use a class on the D side, unless your C++ type uses virtual functions. Classes in D are different from structs, it is not the same as C++ where they are basically the same thing.

I know their differences, classes are reference types and structs are value types in D. I actually don't need this code. I've already wrapped many c code, but have no so much experience with interfacing C++ code. I am trying to understand capabilities of D in interfacing with C++ and its limits. I would like to know why I can interface a template-free class, but not a class template.

On Sunday, 15 March 2020 at 21:27:32 UTC, Ferhat Kurtulmuş wrote:
> On Sunday, 15 March 2020 at 21:16:43 UTC, Arine wrote:
>> On Sunday, 15 March 2020 at 20:53:49 UTC, Ferhat Kurtulmuş
>
>> I wouldn't use a class on the D side, unless your C++ type uses virtual functions. Classes in D are different from structs, it is not the same as C++ where they are basically the same thing.
>
> I know their differences, classes are reference types and structs are value types in D.

That's not the only difference. It's why your code doesn't work.

> I actually don't need this code. I've already wrapped many c code, but have no so much experience with interfacing C++ code. I am trying to understand capabilities of D in interfacing with C++ and its limits. I would like to know why I can interface a template-free class, but not a class template.

You can, you just aren't representing the C++ code properly in D. You don't want class, it means something different in D than in C++.

This works for me, as it is using struct that corresponds to the C++ type. You have to use extern(C++, class), as it is using "class" on the C++ side, which produces a different mangling.

extern(C++, cv){
extern(C++, class) struct Size_(_Tp){
    @disable this();
	~this() { }

    final _Tp area() const;
    final double aspectRatio() const;
    final bool empty() const;

    _Tp width; //!< the width
    _Tp height; //!< the height
}
}

extern(C++){
    cv.Size_!int* createSizeIntWH(int w, int h);
}

void main()
{
	Size_!int* sz = createSizeIntWH(200, 100);
	writeln(sz.width);
}

On Sunday, 15 March 2020 at 22:25:27 UTC, Arine wrote:
> On Sunday, 15 March 2020 at 21:27:32 UTC, Ferhat Kurtulmuş

> extern(C++, cv){
> extern(C++, class) struct Size_(_Tp){
>     @disable this();
> 	~this() { }
>
>     final _Tp area() const;
>     final double aspectRatio() const;
>     final bool empty() const;
>
>     _Tp width; //!< the width
>     _Tp height; //!< the height
> }
> }
>
> extern(C++){
>     cv.Size_!int* createSizeIntWH(int w, int h);
> }
>
> void main()
> {
> 	Size_!int* sz = createSizeIntWH(200, 100);
> 	writeln(sz.width);
> }

This worked for me too. But member functions are still causing linker error. It seems like docs (https://dlang.org/spec/cpp_interface.html) do not cover those situations. Is there any other sources to read for it. Maybe you make a pull request to docs covering C++ interfacing tips in detail.