Linker error: "LNK 2019"

[aba]

I have looked at some of the other topics regarding this error, but I haven't found any that help my problem.

I'm using Visual C++ 2008 and I've been working on a project for a few days and now I'm getting the following error when building:

CRC32.obj : error LNK2019: unresolved external symbol "void __cdecl utils::cpy_array<char>(char * &,char * &,int)" (??$cpy_array@D@utils@@YAXAAPAD0H@Z) referenced in function "public: __thiscall CRC32::CRC32(char *,int)" (??0CRC32@@QAE@PADH@Z)
1>C:\Documents and Settings\aba\My Documents\Visual Studio 2008\Projects\Encryption\Debug\Encryption.exe : fatal error LNK1120: 1 unresolved externals

I've found that removing the line which calls utils::cpy_array, will remove the error, but - of course - that is not a suitable solution.

I've used #pragma once in every .h-file and even added an extra include guard on the file declaring utils::cpy_array.

I might also add that utils::cpy_array is a template function and that it worked until I created another function, which doesn't interfere with utils::cpy_array at all, called string get_flags(char *input[], char *delims).

If anyone has any idea what is coursing this or just a simple way to get around it, I'd be happy to hear :)

[filipe]

Templatized functions must have their definitions included in the header file, not the cpp file. It could be what's causing your problem.

[aba]

It removed the problem, but now I get the following:

1>Main.obj : error LNK2005: "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl utils::get_flags(char * * const,char *)" (?get_flags@utils@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAPADPAD@Z) already defined in CRC32.obj
1>C:\Documents and Settings\aba\My Documents\Visual Studio 2008\Projects\Encryption\Debug\Encryption.exe : fatal error LNK1169: one or more multiply defined symbols found

[filipe]

Sounds like there's two definitions for get_flags(); one in Main.cpp, the other in CRC32.cpp. Either that or the include guard is missing from the header file where get_flags() is declared, but you said all of them are guarded.

[aba]

That was what I read of it as well, but there are no definition of get_flags() in any of those files, only a call from Main.cpp.

However both Main.cpp and CRC32.cpp includes Tools.h (where get_flags() is defined) the guard that I'm using is:

#pragma once

#ifndef __TOOLS__
#define __TOOLS__
.
.
//Declaring and defining get_flags()
.
#endif 

Any idea what might be wrong?

[filipe]

Remove the definitions from Tools.h (except the inlined ones) and move them to a Tools.cpp file.

[aba]

Okay, I got it fixed now :)

All the template functions are defined in the header file as you said.

But what seemed to mess it up was that I also moved a none-template function - guess which :P - so I just moved get_flags() back to the source file again and it works... For now at least.

Thank you for your help.

[aba]

It's weird though that the templates are to be in the header and the regular ones in the source isn't it?

Anywho, it works.

And thanks again.

[filipe]

I find it a bit clumsy too, not to mention it "pollutes" header files. But here's a good explanation of why: http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file

If you're sure only one translation unit will ever use a certain template I don't think there's any technical problems with putting the implementation into a .cpp file, but it's certainly not good practice.