Crossbow

I have a log file class that outputs data to a logfile from a program. Well, I have two source files in a project I want to use this log class in. I have a header that is included by both source files. It gives me some a link error saying it was already defined in one of the source files. I was using extern too. I have tried many things, but none work. How do I make it so that I can use the class in both source files?

Thanks!

BTW, I am using MSVC++ if that matters.

Unregistered

in the top of the class you are including use this:

and at the end

with that you only define the header once, no matter how may times you include it

MrWizard

Do you want to use the same class instance in 2 source files? Or two different objects , in two source files...

__________________
"...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers

Crossbow

I want to use the same class instance in two different source files.

MrWizard

Then you want to use the extern keyword. If you don't know how to use it, just write back and i'll give an example.

Crossbow

I did define it as extern, and then used the class in the two source files, but that game a linking error when I tried to link it. Here is the full compile output:

--------------------Configuration: Normals - Win32 Debug--------------------
Compiling...
3DMath.cpp
Init.cpp
Main.cpp
Linking...
Init.obj : error LNK2005: "public: __thiscall Log::Log(char *,bool)" (??0Log@@QAE@PAD_N@Z) already defined in 3DMath.obj
Init.obj : error LNK2005: "public: __thiscall Log::~Log(void)" (??1Log@@QAE@XZ) already defined in 3DMath.obj
Init.obj : error LNK2005: "public: void __cdecl Log::Logout(char *,...)" (?Logout@Log@@QAAXPADZZ) already defined in 3DMath.obj
Init.obj : error LNK2005: "private: void __thiscall Log::Truncate(void)" (?Truncate@Log@@AAEXXZ) already defined in 3DMath.obj
Init.obj : error LNK2005: "public: void __thiscall Log::FileName(char *,bool)" (?FileName@Log@@QAEXPAD_N@Z) already defined in 3DMath.obj
Init.obj : error LNK2005: "class Log Logfile" (?Logfile@@3VLog@@A) already defined in 3DMath.obj
Main.obj : error LNK2005: "public: __thiscall Log::Log(char *,bool)" (??0Log@@QAE@PAD_N@Z) already defined in 3DMath.obj
Main.obj : error LNK2005: "public: __thiscall Log::~Log(void)" (??1Log@@QAE@XZ) already defined in 3DMath.obj
Main.obj : error LNK2005: "public: void __cdecl Log::Logout(char *,...)" (?Logout@Log@@QAAXPADZZ) already defined in 3DMath.obj
Main.obj : error LNK2005: "private: void __thiscall Log::Truncate(void)" (?Truncate@Log@@AAEXXZ) already defined in 3DMath.obj
Main.obj : error LNK2005: "public: void __thiscall Log::FileName(char *,bool)" (?FileName@Log@@QAEXPAD_N@Z) already defined in 3DMath.obj
Main.obj : error LNK2005: "class Log Logfile" (?Logfile@@3VLog@@A) already defined in 3DMath.obj
Debug/Normals.exe : fatal error LNK1169: one or more multiply defined symbols found
Error executing link.exe.

Normals.exe - 13 error(s), 0 warning(s)

MrWizard

Okay....i'll give you an example. Say you have a main.cpp, a myclass.h, and a second.cpp . .. You define your class in main right? but you want to use it in second? So in your myclass.h you would do this
That would be your header. In your main.cpp *BEFORE* you include myclass.h simply #define MAIN_CPP

This should work. Any questions let me know

__________________
"...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers

Crossbow

I didn't get that to work, but I have attached a simple project with just the class and two functions in different files so you can get it to work and save the changes and post it back up. I wasn't sure whether to use the GLOBAL thing before the prototype in main.cpp or not. Thanks for your time, man.

Crossbow

Here's the file:

Attached Files

logprj.zip (86.5 KB, 13 views)

MrWizard

I do not have the header file Log.h .. If you want to attach that I can help. Just upon looking at it I see one problem with your header file. This is YOUR code
It should be like this....

See if that works.

EDIT: Made a mistake. Whenever you have int's and stuff you can do it with the GLOBAL thing like I did...but with Classes that have constructors...whenever you want it global you have to pass "log.log", true... but when you say you want it extern, you don't say "log.log", true again, you just say Log Logfile; Hope that makes sense....Also, don't declare this in your main.cpp, your header file is already declaring it!!!!