OpenCV 2.3 with Qt 4.3.7

I have succ开发者_运维知识库essfully 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!

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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.

---

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).

---

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.

---

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