Visual C++中向命名空间添加可修改变量引发链接错误求助

问题场景 #

在包含CongregationDatabaseManager命名空间的头文件中添加两个可修改的全局变量后，出现LNK2005多重定义链接错误。

原头文件代码 #

#ifndef CONGREGATION_DATABASE_MANAGER
#define CONGREGATION_DATABASE_MANAGER
#pragma once

namespace CongregationDatabaseManager
{
    int iEditMenuID;
    int iDeleteMenuID;

    static const COLORREF GetAssignedTextColour()
    {
        return theApp.GetNumberSetting(L"Options", L"AssTextColour", PALETTERGB(0, 0, 255));
    }
}
#endif

链接错误信息 #

7>CongregationOptionsDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>CongregationOptionsDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>CongregationSpeakerDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>CongregationSpeakerDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>ChristianLifeMinistryStudentMaterialDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>ChristianLifeMinistryStudentMaterialDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>Meeting Schedule Assistant.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>Meeting Schedule Assistant.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>Meeting Schedule AssistantDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>Meeting Schedule AssistantDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>WeekendMeetingDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>WeekendMeetingDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>PublishersDatabaseDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalEditMenuId" (?iLocalEditMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>PublishersDatabaseDlg.obj : error LNK2005: "int CongregationDatabaseManager::iLocalDeleteMenuId" (?iLocalDeleteMenuId@CongregationDatabaseManager@@3HA) already defined in CongregationDlg.obj
7>D:\My Programs\2022\MeetSchedAssist\Meeting Schedule Assistant\Release\Meeting Schedule Assistant.exe : fatal error LNK1169: one or more multiply defined symbols found

错误原因 #

头文件中直接写int iEditMenuID;属于变量定义，当多个源文件包含该头文件时，每个源文件都会生成该变量的独立副本。链接阶段，链接器发现多个相同符号的定义，就会抛出LNK2005错误。

解决方案 #

遵循C++分离编译规则，将变量的声明和定义分开：

步骤1：修改头文件，仅声明变量 #

在头文件中用extern关键字声明变量，告诉编译器变量的存在，实际定义在其他文件中：

#ifndef CONGREGATION_DATABASE_MANAGER
#define CONGREGATION_DATABASE_MANAGER
#pragma once

namespace CongregationDatabaseManager
{
    // 仅声明变量，不定义
    extern int iEditMenuID;
    extern int iDeleteMenuID;

    static const COLORREF GetAssignedTextColour()
    {
        return theApp.GetNumberSetting(L"Options", L"AssTextColour", PALETTERGB(0, 0, 255));
    }
}
#endif

步骤2：创建单独的.cpp源文件，定义变量 #

新建一个比如CongregationDatabaseManager.cpp的文件，在其中定义变量，提供唯一的实例：

#include "CongregationDatabaseManager.h"

namespace CongregationDatabaseManager
{
    // 变量的实际定义，仅此处存在一份
    int iEditMenuID;
    int iDeleteMenuID;
}

额外说明 #

如果需要给变量初始化默认值，直接在.cpp的定义处添加即可，比如：

int iEditMenuID = 0;
int iDeleteMenuID = 0;

这样修改后，所有包含头文件的源文件都会共享.cpp中定义的唯一变量实例，链接错误就会消除。

内容的提问来源于stack exchange，提问作者Andrew Truckle