c++ - creating global variables causes linker error

Question

I have an MFC application AVT_testapp, and in the header file (AVT_testappDlg.h) I am trying to create a variable outside of all functions, classes, etc. in order to make it global. Whenever I try to do this though (say I try int x = 7), I get the error:

1>AVT_testappDlg.obj : error LNK2005: "int x" (?x@@3HA) already defined in 
    AVT_testapp.obj
1>....inx64DebugAVT_testapp.exe : fatal error LNK1169: one or more 
    multiply defined symbols found

Everything I have found on google says "just add header guards". AVT_testappDlg has 6 #include's, and each of them has header guards.

What else could be causing these errors when creating global variables?

EDIT: Here is the beginning of my header file,

#pragma once

#include "../../src/CoreUtils/nierr.h"
#include "....srcCoreUtilsStringHelpers.h" //includes windows.h
#include "afxwin.h"
#include "afxcmn.h"
#include "IFrameObserver.h"
#include "c:Program Files (x86)Microsoft SDKsWindowsv7.0AIncludeGdiPlusHeaders.h"
//#include <fstream>
//#include <windows.h>

int x = 7;

using namespace AVT::VmbAPI;


//////////////////////////////////////////////////////////////////////////
//////////  MyObserver class   ///////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////
class MyObserver : public IFrameObserver
{
private:
    MyObserver( MyObserver& );

    MyObserver& operator=( const MyObserver& );    

public:

    VmbUchar_t* imageData;

            //...
            //...
            //...
            //...

//that's the end of the relevant stuff

Answer 1

You cannot define variables at namespace level in a header. In general it is best not to have global variables, but if you need to you should provide only a declaration in the header and the definition in a single .cpp:

//header
extern int i;

//cpp
int i;

The problem with your code is not related to header guards. Header guards ensure that a header is parsed only once in each translation unit. Lack of header guards causes compiler errors, where the compiler sees, say for example a class, defined multiple times in the same translation unit after preprocessing. In your case the error is a linker error LNK2005, and it means that the same symbol was defined in multiple translation units (in your case in each translation unit that includes the header with the definition).