创建全局变量会导致链接器错误

2022-01-11 00:00:00 global-variables compiler-errors linker c++ mfc

我有一个 MFC 应用程序 AVT_testapp，并且在头文件 (AVT_testappDlg.h) 中，我试图在所有函数、类等之外创建一个变量，以使其成为全局变量.每当我尝试这样做时(比如我尝试 int x = 7)，我都会收到错误消息:

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

我在 google 上找到的所有内容都说只需添加标头保护". AVT_testappDlg 有 6 个#include，每个都有标头保护.

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

在创建全局变量时还有什么可能导致这些错误?

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

这是我的头文件的开头，

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

推荐答案

您不能在标题中的命名空间级别定义变量.一般来说，最好不要有全局变量，但如果需要，您应该只在标头中提供 声明，并在单个 .cpp 中提供定义:

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;

您的代码问题与标头保护无关.标头保护确保标头在每个翻译单元中仅解析一次.缺少标头保护会导致 compiler 错误，编译器会看到，例如一个类，在预处理后在同一个翻译单元中定义了多次.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(在您的情况下，在每个翻译单元中都包含带有定义的标题).

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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