WinForm application : Capture videostream in a new form. Error LNK2005!

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Hello,

I want to realize this thing:
Form1(Button_Click)-> Form2(Butto_Click)-> Video preview from webcam.

what I already can:
Form1(Button_Click)-> Video Preview.

I easily make form2.h under form1 and CamShow.h under form2.
But i got this error which says:
error LNK2005: "public: __thiscall WebcamShow::CamShow::CamShow(void)" (??0CamShow@WebcamShow@@QAE@XZ) is already in Form2.obj defined.
(and all the methods i declared in CamShow are mentioned just the same)

Codes for CamShow.h:

C++

#include <DShow.h>
#include <Windows.h>
#include <comdef.h>

#define WM_GRAPHNOTIFY  WM_APP+1
enum VIDEOSTATUS {Stopped, Paused, Running, Init};
namespace WebcamShow
{
	public class CamShow
	{
	public: CamShow(); 
		~CamShow();

	public:		HRESULT CamShow::InitComLib();
			HRESULT CamShow::CaptureVideo(int);
			......
	public: 
			IGraphBuilder			*pGraphBuilder;
			ICaptureGraphBuilder		*pCGB;
			ICaptureGraphBuilder2		*pCGB2;
			IVideoWindow			*pVW;

			IMediaControl			*pMC;
			IMediaEventEx			*pME;
                        ......
			
	};

	CamShow::CamShow()
	{		
			pGraphBuilder				= NULL; 
			pCGB					= NULL;
			pCGB2					= NULL;
			pVW					= NULL;

			pMC					= NULL;
			pME					= NULL;

			......
	}
	CamShow::~CamShow() {}

	HRESULT CamShow::InitComLib()
	{
		HRESULT hr = NULL;
		hr = CoInitialize(NULL);
		return hr;
	}
        ......

Form1:

C++

#pragma once
#include "Form2.h"
......

And Form2:

C++

#pragma once
#include "CamShow.h"
......

i think the problem is about duplicated declarations. And i've read the msdn explaination for this error, which i don't quite understand.
How can I solve this?

Thanks!!!

Posted 3-Oct-12 23:02pm

christmars

Updated 3-Oct-12 23:16pm

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Jochen Arndt · Accepted Answer · 2012-10-03T23:32:00

Solution 1

Your your first code block CamShow.h is a header file containing not only declarations but also code that will be compiled each time the file is included. So you will get the same code compiled multiple times and stored in different object files. The linker then generates the error.

You should move the code sections (constructor, destructor, functions) into an own source file CamShow.cpp. Optionally you may declare short functions as inline.

Posted 3-Oct-12 23:32pm

Jochen Arndt

Comments

christmars 4-Oct-12 8:35am

Hi Jochen,

i used inline but another problem happens: i can't create a class. It tells me:
error LNK2005: "class WebcamShow::CamShow test" (?test@@3VCamShow@WebcamShow@@A) is already in Form2.obj defined.
And when i do the .cpp, there is the same problem!

Jochen Arndt 4-Oct-12 9:00am

Did you have a global CamShow object in some header file (a line containing 'CamShow test;')? Then this would result in this error. Depending on your project the object should be not global but part of another class (Form1 and/or Form2 or the app/mainframe/document class).

It seems that you have mixed up definitions (code implementation) and declarations.

But it is hard to help without knowing more details. Using the provided details, I suggest to add the CamShow object as a member to your app, main frame, or document class and provide functions to access the member from your forms.

christmars 4-Oct-12 9:30am

No, i didn't.
But it works when I just put the class definition within the button_click event.
I definitated it after "using namespace WebcamShow;" earlier and it had error.

Jochen Arndt 4-Oct-12 12:22pm

I still can not imagine how you use the class. But I just recognized that you used the '#pragma once' directives followed by include statements. Are these pragmas in the .cpp source files? They are for usage with header files and you should add one to your CamShow.h file.

christmars 4-Oct-12 15:25pm

I made the declaration of a class(CamShow test) within button_click event. It seems like that the declaration outside or in public sections caused error. But it's not good when the defination of the class is within the event block. Because i can't access it all over in the form2.

To your question: no, my .cpp doesn't have pragma once in it. So does CamShow.h. I'll try to add this in .h to see, if this matters.

christmars 5-Oct-12 3:06am

I've tried to add the "#pragma once" to CamShow.h file, but this doesn't make sense. A class(CamShow test;) still can only be declared within a event section and only be useful there.
I'm not able to create a CamShow class, that across form2 to be used.

Jochen Arndt 5-Oct-12 3:16am

Without seeing your code it is really hard to guess what may be wrong.
I will try to explain what happens:
The code of your CamShow class is compiled multiple times and stored multiple times in one or more object files. Then the linker complains about this because he did not know which of the code shouild be used (called).

To simplify things you may temporarily remove the WebcamShow namespace. Regarding the namespace: Did you have 'using WebcamShow' directives or did you use the full name (WebcamShow::CamShow) when creating an object?

christmars 5-Oct-12 4:06am

hi, i'll make a new question with detailed codes to let you see the current state. And explain what is the trouble. Maybe you'll see my error. Thanks really a lot!

christmars 5-Oct-12 7:25am

so i've taken the "using namespace WebcamShow" away, but that leads to no changes. Please see the relatively more detailed version:
http://www.codeproject.com/Questions/471204/WinForm-app-Capture-a-videostream-in-a-new-form-Er

Jochen Arndt 5-Oct-12 7:47am

I just saw it. And the problem seems to be clear:

Form1.cpp/Form1.h includes Form2.h and creates a global CamShow object (in namespace LOSCH).
Form2.cpp includes also Form2.h and creates another CamShow object with the same name in the same namespace.
This can't be handled by the linker.

You must make the CamShow object a member of each class where it is used (Form1 and Form2) or use only one global object.

I'm not very familar with .NET (more C++), so I don't know if putting code into header files is common with .NET. With C++, code is rarely used in header files (exception: inline code and templates). I suggest to put the code blocks of the Form1 and Form2 functions into the source files. But this would not solve the problem while the CamShow object is global and defined multiple times.

christmars 5-Oct-12 10:09am

Thanks for that!! :) I removed the #include Form2.h in Form2.cpp, this time it's ok to make a global class. But i am not pretty sure if this is a stable solution or not.

Jochen Arndt 5-Oct-12 10:17am

Form2.h belongs to Form2.cpp, so it should be there. See the solution by Richard for the new question. That's the way how it should be done (I saw your comment, but it should work; did you tried a rebuild all?)

christmars 6-Oct-12 4:36am

I acturally did. And that was what i've done before i wrote question 2. But the error is the same.
But in the win form applications, the Form.cpp always just has StdAfx.h and Form.h, no more codes. That means maybe we only use .h file. It seems like they are not needed. What's your opinion?

Jochen Arndt 8-Oct-12 3:05am

When you derive a class using the wizard, you will get a .cpp and .h file. It's up to you to extend the class with your code. The code (definitions) is put into the cpp file and the declarations are put into the .h file. Please understand the difference between definition and declaration:

A declaration (of a class, a function, or a variable) tells the compiler how to handle the item when used inside the code. The compiler must know the declaration when compiling the implementation (e.g. the body of a classes function) and when using items (e.g. using a class somewhere). Declarations will not produce any output code.

A definition is the implementation of declared items (e.g. the bodies of the functions of a class).

If you have something like 'CMyClass m_Class;' in a header file inside a class declaration, it is a declaration of a member variable of the class.

But if the same line is outside a class declaration, it is a definition that will be compiled and produces code placed in the corresponding output file (the object file with the name of the cpp file that included the header file.)

If you still got the error, you probably still have some definition (e.g. 'CMyClass m_Class;') inside a header file outside a class declaration.

christmars 8-Oct-12 6:22am

Thank you for that! I also thought what I did here is only a declaration of a "m_Class". Because when defination are duplicated is the error like this: new defination of ...
But you see the system tells me another error:
error LNK2005: "class WebcamShow::CamShow test" (?test@@3VCamShow@WebcamShow@@A) is already in Form2.obj defined.

So maybe I mixed up the defination and declaration due to the word "defined". But I really appreciate what you pointed :

Form1.cpp/Form1.h includes Form2.h and creates a global CamShow object (in namespace LOSCH).
Form2.cpp includes also Form2.h and creates another CamShow object with the same name in the same namespace.
This can't be handled by the linker.

You must make the CamShow object a member of each class where it is used (Form1 and Form2) or use only one global object.

And I've tested an easy programm with windows forms application. Open the next form with a button click(Form1->Form2->Form3). That I just take all the automatically created Form1.cpp, Form2.cpp and Form3.cpp away. And that has no influence on my programm. In conclusion, I think the Form.cpp is able to be cancled when they are not used.

Jochen Arndt 8-Oct-12 6:53am

The error says:
There is a global object 'WebcamShow::CamShow test' in the module Form2.obj (created when compiling Form2.cpp). But there is another global object with the same name elsewhere (unfortunately the linker does not tell us where; it may be in the same module).

Please see Maxim Kartavenkov's comment in the other thread to use global objects (declaration with 'extern' in a header file and definition in the corresponding source file). But global objects are bad C++ programming style. The common solution for this is to add such objects as member variables to classes that can be accessed from every part of your program like the application or main frame class.

You may exclude the generated source files from the project when they would not contain any code. But then you usually don't need a derived class but can use the base class instead. In your case you have at least the click handlers which should be placed in the source files.