OpenCV 2.3 with Qt 4.3.7 - c++

Question

I have successfully build and ran both Qt 4.3.7 and OpenCV 2.3 with Qt enabled. When I start a window using:
cvNamedWindow( "video", 0 );
I successfully load a full Qt interface! wonderful :)
However!! when I use the command
void callbackButton(int state, void* userdata){
int x;
x=3;
}
cvCreateButton(nameb2,callbackButton,nameb2,CV_CHECKBOX,0);
I get the error message
error LNK2001: unresolved external symbol _cvCreateButton
I don't understand as the Qt interface already has lots of buttons on it? could someone please explain what I am missing from the include that could cause this?
Thanks!

Answer 1

You use the wrong parameters for to call to cvCreateButton. According to the documentation here the signature of the function is
cvCreateButton(const char* button_name CV_DEFAULT(NULL), CvButtonCallback on_change CV_DEFAULT(NULL), void* userdata CV_DEFAULT(NULL), int button_type CV_DEFAULT(CV_PUSH_BUTTON), int initial_button_state CV_DEFAULT(0)
and sample calls are:
cvCreateButton(NULL,callbackButton);
cvCreateButton("button2",callbackButton,NULL,CV_CHECKBOX,0);
cvCreateButton("button3",callbackButton,&value);
cvCreateButton("button5",callbackButton1,NULL,CV_RADIOBOX);
cvCreateButton("button6",callbackButton2,NULL,CV_PUSH_BUTTON,1);
and the declaration of the callback function has to be:
CV_EXTERN_C_FUNCPTR( *CvButtonCallback)(int state, void* userdata));
You get a linking error and not a compiler error because cvCreateButton has extern "C" linkage - which means that parameters cannot be checked at compile time.

Answer 2

I solved this issue by calling the function cv::createButton instead of cvCreateButton (which is if I am correct the way to call methods in OpenCV2).

Answer 3

The third argument must be a void*. Change to:
cvCreateButton(nameb2,callbackButton,NULL,CV_CHECKBOX,0);
and it will work.
Edit
The statement above was given an error.
The third needed argument is a "void *" - this is compatible with anything and thus neither C nor C++ should have a problem with what you were providing. You can not raise a linker error with that.
The only reason a linker error can be raised by coding is when you don't use prototypes (forgot to use the header file) in C++ and then C++ creates a mangled name on its own that wont be part of any library. In such a case the compiler will first tell you with a warning at compile time that you are missing the prototype (for C and C++) - and then the linker will probably raise an error (for c++ only).
If you don't see a prototype warning from the compiler then that is not your problem.

Answer 4

This is a linking error.
Try to add the opencv .lib file (or files) to the project libraries path.
This may help : VS2010 OpenCV.
Edit
Refined problem: Even if adding any OpenCV libary to your project the linking will fail.
Reason: The symbol is often simply not there in the libraries.
Solution: You have to change a few settings and compile them on your own.
See also: openCV 2.2 createButton LNK 2019 error in Visual Studio 2010