[C++] Visual Studio 2010's strange “warning LNK4042”


Answers

I had a similar problem with linker warning LNK4042: object specified more than once; extras ignored. In my case Visual Studio was trying to compile both header and source files with the same name - MyClass.h and MyClass.cpp. It happened because I renamed .cpp file to .h and Visual Studio got confused. I noticed the problem by looking at the compiler logs in the Debug directory. To resolve just remove .h file from the project then add it again.

Question

I've just been beaten (rather hardly) on the head by some non-trivial warning from Visual Studio 2010 (C++).

The compilation gave the following output:

1 Debug\is.obj : warning LNK4042: object specified more than once; extras ignored
1 Debug\make.obj : warning LNK4042: object specified more than once; extras ignored
1 Debug\view.obj : warning LNK4042: object specified more than once; extras ignored
1 identity.obj : error LNK2019: unresolved external symbol void __cdecl test::identity::view(void) (?view@identity@test@@YAXXZ) referenced in function void __cdecl test::identity::identity(void) (?identity@0test@@YAXXZ)
1 identity.obj : error LNK2019: unresolved external symbol void __cdecl test::identity::make(void) (?make@identity@test@@YAXXZ) referenced in function void __cdecl test::identity::identity(void) (?identity@0test@@YAXXZ)
1 range.obj : error LNK2019: unresolved external symbol void __cdecl test::range::is(void) (?is@range@test@@YAXXZ) referenced in function void __cdecl test::range::range(void) (?range@0test@@YAXXZ)

Linker errors are always a pain to debug... but there were unresolved references, and so I checked... but the source is well-formed... and finally it hit me:

My folder hierarchy looks like so:

src/
  identity/
    is.cpp
    make.cpp
    view.cpp
  range/
    is.cpp
    make.cpp
    view.cpp

and so does the hierarchy in the Solution (I always set it up so that it mimicks the "real" folder structure).

And the diagnostic outputs:

Debug\is.obj
Debug\make.obj
Debug\view.obj

Along with a warning which says that the .obj has been passed twice to the linker and that one will be ignored.

Search no more: Visual has neatly flatten my folder hierarchy, and therefore is unable to neatly compile the source.

At the moment, I am simply thinking of renaming the files, that should cover the issue...

... but is there a way to have Visual Studio NOT flatten the file hierarchy ?




Visual Studio 2010's strange “warning LNK4042”

Just wanted to cross post what I believe to be the answer, if you open the properties for the entire project, and the change the value under C/C++ -> Output Files -> "Object File Name" to be the following:

$(IntDir)/%(RelativeDir)/

Under VS 2010, I believe this will disambiguate all of the object files (as I believe windows won't let you under any crazy circumstances have two files with the same names in the same directory). Please also check out the details here.




VS2008: Can I build a project with 2 CPP files of the same name in different folders?

Right click both/either .cpp files > properties > C/C++ > Output Files > Object File Name > set a custom name. e.g. if both files are named MyFile.cpp in folder A and another in folder B, you can set the output to be AMyFile and BMyFile.

Alternatively, you can also use a macro to prefix the object names with the immediate parent folder name (i.e. using $(IntDir)\$(SafeParentName)$(SafeInputName)). If this is not enough (e.g. you have A/B/MyFile.cpp and C/B/MyFile.cpp) and you don't mind having some object files cluttering your source tree, you can also use $(InputDir)\ which will put the object files in the same folder as the source file.

the cpp files will then be compiled into two different object files..

enjoy!

Update for VS2010: There is a better solution in VS2010, check it out here. Thanks to n1ck's comment

btw, if the contents have the same name, do you separate them using different namespaces?

namespace A { // in folder A
    class CMyFile {};
};

namespace B{ // in folder B
    class CMyFile {};
};

// client.cpp
#include "A/MyFile.h"
#include "B/MyFile.h"
int main() {
    A::CMyFile aMyFile;
    B::CMyFile bMyFile;
    return 0;
}

I don't know if it matters but it's definitely clearer to human : D




This is easy to fix in the IDE. Click the first file in the folder, Shift+Click the last file so all of them are selected. Right-click, Properties, C++, Output Files. Change the Object File Name from $(IntDir)\ to, say, $(IntDir)\Univariate\. You can repeat for the Multivariate file group although that's not strictly necessary.




Right-click the .cpp file in the Solution Explorer window, Properties, C/C++, Output Files, Object File Name setting. The default is $(IntDir)\, that's what is doing the flattening. All the .obj file will go into $(IntDir), the "Debug" directory in the debug configuration.

You can change the setting, say $(IntDir)\is2.obj. Or select all the files from one group (use Shift+Click) and change the setting to, say, $(IntDir)\identity\

Or you can change the .cpp filename so that .obj files don't overwrite each other. Having files with the exact same name in two directories is a bit odd.

Or you can create multiple projects, creating, say, .lib projects for the files in identity and range. Commonly done in makefile projects for example. That does however make managing the compile and link settings more of a hassle unless you use project property sheets.




Do you really WANT two different but same-named files in the same project?