Thread Subject: Cast calllib result to a struct pointer?

I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem() function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#cvGetSeqElem) like this:

cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);

where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The MATLAB signature is:

[cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)

On Nov 14, 7:59 am, "John Lyle" <mathwo...@myallit.com> wrote:
> I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem() function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#...) like this:
>
> cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
>
> where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The MATLAB signature is:
>
> [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)

are you trying to typecast in matlab? better do it in c/c++ if you
can.
R

Yes I am, is that not possible? I'm trying to use the OpenCV DLLs directly in MATLAB without writing any C++ wrapper code.

roger <northsolomonsea@gmail.com> wrote in message <5badc0bd-f0dd-44e8-9153-b0975f7ffe2b@l2g2000yqd.googlegroups.com>...
> On Nov 14, 7:59?am, "John Lyle" <mathwo...@myallit.com> wrote:
> > I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem() function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#...) like this:
> >
> > cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
> >
> > where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The MATLAB signature is:
> >
> > [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)
>
> are you trying to typecast in matlab? better do it in c/c++ if you
> can.
> R

On Nov 15, 5:34 am, "John Lyle" <mathwo...@myallit.com> wrote:
> Yes I am, is that not possible? I'm trying to use the OpenCV DLLs directly in MATLAB without writing any C++ wrapper code.
>
>
>
> roger <northsolomon...@gmail.com> wrote in message <5badc0bd-f0dd-44e8-9153-b0975f7ff...@l2g2000yqd.googlegroups.com>...
> > On Nov 14, 7:59?am, "John Lyle" <mathwo...@myallit.com> wrote:
> > > I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem() function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#...) like this:
>
> > > cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
>
> > > where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The MATLAB signature is:
>
> > > [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)
>
> > are you trying to typecast in matlab? better do it in c/c++ if you
> > can.
> > R

Typecasting in matlab is very basic and afaik you can't cast non
(matlab) base types, apart from that it's inefficiently implemented.
Better write a small wrapper, if typecasting is all it does it will be
easy enough.

OK thanks for your help

roger <northsolomonsea@gmail.com> wrote in message <5601f08d-e6c6-4fa1-8b24-f8f83c6c0c91@g23g2000yqh.googlegroups.com>...
> On Nov 15, 5:34?am, "John Lyle" <mathwo...@myallit.com> wrote:
> > Yes I am, is that not possible? I'm trying to use the OpenCV DLLs directly in MATLAB without writing any C++ wrapper code.
> >
> >
> >
> > roger <northsolomon...@gmail.com> wrote in message <5badc0bd-f0dd-44e8-9153-b0975f7ff...@l2g2000yqd.googlegroups.com>...
> > > On Nov 14, 7:59?am, "John Lyle" <mathwo...@myallit.com> wrote:
> > > > I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem() function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#...) like this:
> >
> > > > cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
> >
> > > > where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The MATLAB signature is:
> >
> > > > [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)
> >
> > > are you trying to typecast in matlab? better do it in c/c++ if you
> > > can.
> > > R
>
> Typecasting in matlab is very basic and afaik you can't cast non
> (matlab) base types, apart from that it's inefficiently implemented.
> Better write a small wrapper, if typecasting is all it does it will be
> easy enough.

The best way to do this is with one or more aliases to the function cvGetSeqElem.

Generate a MATLAB file using loadlibrary with the mfilename option and edit the file to add aliases for the function with the return
types you are going to use.

See the bottom of the loadlibrary documentation for an example using mxGetNumberOfDimensions.

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/loadlibrary.html

Phil

"John Lyle" <mathworks@myallit.com> wrote in message news:hdlkfp$773$1@fred.mathworks.com...
>I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem()
>function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#cvGetSeqElem) like this:
>
> cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
>
> where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The
> MATLAB signature is:
>
> [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)

Thanks Philip, that's exactly what I was looking for, works perfectly

"Philip Borghesani" <philip_borghesani@mathworks.spam> wrote in message <hdro7g$2t4$1@fred.mathworks.com>...
> The best way to do this is with one or more aliases to the function cvGetSeqElem.
>
> Generate a MATLAB file using loadlibrary with the mfilename option and edit the file to add aliases for the function with the return
> types you are going to use.
>
> See the bottom of the loadlibrary documentation for an example using mxGetNumberOfDimensions.
>
> http://www.mathworks.com/access/helpdesk/help/techdoc/ref/loadlibrary.html
>
> Phil
>
> "John Lyle" <mathworks@myallit.com> wrote in message news:hdlkfp$773$1@fred.mathworks.com...
> >I have a DLL function that returns a char* but I need to cast this to a struct*. Specifically I am trying to use the cvGetSeqElem()
> >function from OpenCV (http://opencv.willowgarage.com/documentation/dynamic_structures.html#cvGetSeqElem) like this:
> >
> > cvRect = calllib('cxcore', 'cvGetSeqElem', cvSeq, 1);
> >
> > where cvSeq is of type CvSeqPtr. This returns a string. How do I now cast this to a CvRect* to read the returned values? The
> > MATLAB signature is:
> >
> > [cstring, CvSeqPtr] = cvGetSeqElem(CvSeqPtr, int32)
>