创建全局变量导致链接器错误 [英] creating global variables causes linker error

问题描述

我有一个MFC应用程序AVT_testapp，并在头文件（AVT_testappDlg.h）我试图创建一个变量所有的函数，类等之外，以使其全局。每当我尝试这样做（比如我尝试 int x = 7 ），我得到的错误：

  1> AVT_testappDlg.obj：error LNK2005：已在
 AVT_testapp.obj 
 1> .. \中定义的int x（？x @@ 3HA） .\bin \x64\Debug\AVT_testapp.exe：致命错误LNK1169：一个或多个
多重定义的符号找到
  

我在google上找到的所有内容都表示只是添加标题保护。 AVT_testappDlg有6个#include，

$ b

编辑：这是开始的时候的头文件

  #pragma once 
 
 #include../../src /CoreUtils/nierr.h
 #include..\..\src\CoreUtils\StringHelpers.h//包括windows.h 
 #includeafxwin.h
 #includeafxcmn.h
 #includeIFrameObserver.h
 #includec：\Program Files（x86）\Microsoft SDKs\Windows \v7.0A\\ \\Include\GdiPlusHeaders.h
 //＃include< fstream> 
 //＃include< windows.h> 
 
 int x = 7; 
 
使用命名空间AVT :: VmbAPI; 
 
 
 //////////////////////////////////////// ////////////////////////////////////// 
 ////////// MyObserver类/////////////////////////////////////////// 
 / ////////////////////////////////////////////////// /////////////////////// 
 class MyObserver：public IFrameObserver 
 {
 private：
 MyObserver（MyObserver& ;）; 
 
 MyObserver& operator =（const MyObserver&）; 
 
 public：
 
 VmbUchar_t * imageData; 
 
 // ... 
 // ... 
 // ... 
 // ... 
 
这是相关内容的结束
  

解决方案

>定义变量在标题中的命名空间级别。一般来说，最好不要有全局变量，但如果你需要，你应该在标题和单个.cpp中的定义中提供一个声明：

  // header 
 extern int i; 
 
 // cpp 
 int i; 
  

您的代码的问题与标题卫is无关。标题保护确保每个转换单元只解析一次标题。缺少标头保护程序会导致编译器错误，编译器会在预处理之后在同一个翻译单元中查看多次定义的类，例如一个类。在您的情况下，错误是链接器错误 LNK2005 ，这意味着在多个翻译单元中定义了相同的符号（在每个翻译单元中包括带定义的标题）。

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>..\..\bin\x64\Debug\AVT_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 "..\..\src\CoreUtils\StringHelpers.h" //includes windows.h
#include "afxwin.h"
#include "afxcmn.h"
#include "IFrameObserver.h"
#include "c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\GdiPlusHeaders.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

解决方案

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).

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