What really causes "Already defined in *.obj" error, and how to prevent it?

Question

What really causes "Already defined in *.obj" error, and how to prevent it?

JWG 146

I would like to understand the real cause of this error, and best practice in preventing it.

I have an application App.cpp, which includes a main class - Main.h which Main.cpp includes from, but I have an Init.h for all the necessary system and other header file, which Main.cpp will include.

A code like this, in Init.h

constexpr auto MAX_LOADSTRING = 100;

WCHAR szTitle[MAX_LOADSTRING];

causes an error.

LNK 2005 wchart* szTitle(?szTitle@@3PA_WA) already defined in App.obj LNK1169 One or more multiply defined symbols found

I can make WCHAR static, and the error goes away

static WCHAR szTitle[MAX_LOADSTRING];

Or, I can move the code to App,cpp.

However, I don't think this is the solution, since, the error occurs on a simple variable like

bool test = false;

...and any other variable which isn't constant. I don't need these variables constant.

Also, it's more tidy and organized to keep all my global variables in a separate header file.

The message also says, see previous definitions of IDI_APP, which I noticed in the RC and Resource file, but I don't understand anything about that.

I'll like to understand what's relly happening, so that I can avoid running into the problem in the future. Thanks

David Lowndes 2,630 Reputation points MVP

2023-06-10T18:33:26.87+00:00

Declare your global variables as extern in the header file, and once (without extern) in a source file.
RLWA32 49,301 Reputation points

2023-06-10T19:53:08.66+00:00

The code has violated what is known as the "one definition rule." An internet search for that phrase will yield many hits that you can explore. Microsoft's explanation can be found at Translation units and linkage.
JWG 146 Reputation points

2023-06-11T02:20:28.3133333+00:00

Thanks. I understand.

There's just one thing I don't understand, but I 'll make that a new question.

Actually, I could ask here, since it seems related.

I get many warnings, the macro IDI_(APPLICATION-NAME) is being redefined.

if I go to the application's .rc file, and do

#ifdef IDI_(APPLICATION-NAME)

#undef IDI_(APPLICATION-NAME)

#endif

Its undefined.

Then, I do

#ifndef IDI_(APPLICATION-NAME)

#define IDI_(APPLICATION-NAME)

#endif

I get a warning, IDI_(APPLICATION-NAME) : redefined

Where is this macro being redefined? Would this be due to how my header files are included? Or somewhere else?
RLWA32 49,301 Reputation points

2023-06-11T06:22:11.6266667+00:00

Since I don't know how you created your application and it's related resources I cannot give you an exact answer. However, resource IDs are usually defined in a resource.h header file. If you create a new Windows Desktop Application using the Visual Studio template you can examine it and see how resources are typically handled.
JWG 146 Reputation points

2023-06-11T15:55:00.5233333+00:00
Since I don't know how you created your application and it's related resources I cannot give you an exact answer.

Init.h hold the necessary header files and global variables. It includes framework.h and App.h, which would normally be done by the Main.cpp.

#include "framework.h"

#include "App.h"

MainClass.h declares the class and its members. It includes Init.h

#include "Init.h"

MainClass.cpp holds the definitions of the class.

#include "MainClass.h"

The main file App.cpp inherits from the MainClass header file.

#include "MainClass.h"

However, resource IDs are usually defined in a resource.h header file. If you create a new Windows Desktop Application using the Visual Studio template you can examine it and see how resources are typically handled.

Yes, I meant Resource.h, not the .rc file. It's the resource file where I did

#ifdef IDI_(APPLICATION-NAME)

#undef IDI_(APPLICATION-NAME)

#endif

#ifndef IDI_(APPLICATION-NAME)

#define IDI_(APPLICATION-NAME)

#endif

Since the first line of code

#undef IDI_(APPLICATION-NAME)

executed, it suggests to me that IDI_(APPLICATION-NAME) is being defined elsewhere.

Also, when using the above code, I get only one warning of "IDI_(APPLICATION-NAME) redefined", rather than three which I get when not using guards.

I have been working with the Windows Desktop Application, and am familiar with the files used... at least those in the Solution explorer. That's about it.

I don't know how the .rc file works under the hood.

I see the application ID used for icon and cursor, in the window class, but I don't have a clue what, where, or how the program interacts with that.

I just know it's defined in the resource header, that's all.
RLWA32 49,301 Reputation points

2023-06-11T19:08:52.4033333+00:00

@JWG At this point I suggest that you ask a new question and provide enough detail about your code to enable us to determine what is happening. General narrative descriptions don't provide much information. After all, this discussion has strayed far from the linker problem posed in the initial question which I suspect has been answered to your satisfaction
JWG 146 Reputation points

2023-06-11T20:43:22.1766667+00:00

It's okay.

Your answer

David Lowndes 2,630 Reputation points MVP

2023-06-10T18:33:26.87+00:00

Declare your global variables as extern in the header file, and once (without extern) in a source file.
RLWA32 49,301 Reputation points

2023-06-10T19:53:08.66+00:00

The code has violated what is known as the "one definition rule." An internet search for that phrase will yield many hits that you can explore. Microsoft's explanation can be found at Translation units and linkage.
JWG 146 Reputation points

2023-06-11T02:20:28.3133333+00:00

Thanks. I understand.

There's just one thing I don't understand, but I 'll make that a new question.

Actually, I could ask here, since it seems related.

I get many warnings, the macro IDI_(APPLICATION-NAME) is being redefined.

if I go to the application's .rc file, and do

#ifdef IDI_(APPLICATION-NAME)

#undef IDI_(APPLICATION-NAME)

#endif

Its undefined.

Then, I do

#ifndef IDI_(APPLICATION-NAME)

#define IDI_(APPLICATION-NAME)

#endif

I get a warning, IDI_(APPLICATION-NAME) : redefined

Where is this macro being redefined? Would this be due to how my header files are included? Or somewhere else?
RLWA32 49,301 Reputation points

2023-06-11T06:22:11.6266667+00:00

Since I don't know how you created your application and it's related resources I cannot give you an exact answer. However, resource IDs are usually defined in a resource.h header file. If you create a new Windows Desktop Application using the Visual Studio template you can examine it and see how resources are typically handled.
JWG 146 Reputation points

2023-06-11T15:55:00.5233333+00:00

Since I don't know how you created your application and it's related resources I cannot give you an exact answer.

Init.h hold the necessary header files and global variables. It includes framework.h and App.h, which would normally be done by the Main.cpp.

#include "framework.h"

#include "App.h"

MainClass.h declares the class and its members. It includes Init.h

#include "Init.h"

MainClass.cpp holds the definitions of the class.

#include "MainClass.h"

The main file App.cpp inherits from the MainClass header file.

#include "MainClass.h"

However, resource IDs are usually defined in a resource.h header file. If you create a new Windows Desktop Application using the Visual Studio template you can examine it and see how resources are typically handled.

Yes, I meant Resource.h, not the .rc file. It's the resource file where I did

#ifdef IDI_(APPLICATION-NAME)

#undef IDI_(APPLICATION-NAME)

#endif

#ifndef IDI_(APPLICATION-NAME)

#define IDI_(APPLICATION-NAME)

#endif

Since the first line of code

#undef IDI_(APPLICATION-NAME)

executed, it suggests to me that IDI_(APPLICATION-NAME) is being defined elsewhere.

Also, when using the above code, I get only one warning of "IDI_(APPLICATION-NAME) redefined", rather than three which I get when not using guards.

I have been working with the Windows Desktop Application, and am familiar with the files used... at least those in the Solution explorer. That's about it.

I don't know how the .rc file works under the hood.

I see the application ID used for icon and cursor, in the window class, but I don't have a clue what, where, or how the program interacts with that.

I just know it's defined in the resource header, that's all.
RLWA32 49,301 Reputation points

2023-06-11T19:08:52.4033333+00:00

@JWG At this point I suggest that you ask a new question and provide enough detail about your code to enable us to determine what is happening. General narrative descriptions don't provide much information. After all, this discussion has strayed far from the linker problem posed in the initial question which I suspect has been answered to your satisfaction
JWG 146 Reputation points

2023-06-11T20:43:22.1766667+00:00

It's okay.

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What really causes "Already defined in *.obj" error, and how to prevent it?

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