Visual Studio 2010's strange “warning LNK4042”

问题

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 ?

回答1:

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.

回答2:

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.

回答3:

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.

回答4:

Right click on header file -> Property -> ItemType (select C/C++ Header). Do the same with Cpp file but select C/C++ COmpiler (it's work for me)

回答5:

I use $(IntDir)\%(Directory)\ under C/C++ -> Output Files -> "Object File Name".

回答6:

I had this problem with stdafx.cpp. Somehow stdafx.cpp got duplicated, so there was a second StdAfx.cpp (mind the different case).

After I removed the StdAfx.cpp everything worked fine!

Using VS 2010.

回答7:

Alternatively to deleting and making a new file you can change the compile/include settings.

Go to your project.vcxproj file, open it with an editor, find the html like line <ItemGroup>.

It should look something like:

<ItemGroup>
<ClCompile Include="implementation.cpp" />
</ItemGroup>

and

<ItemGroup>
<ClInclude Include="declaration.hpp" />
</ItemGroup>`

Assuming your implementation files are .cpp and your declarations are .hpp. Make sure your all your implementation files are listed between the first section if you have more then one and likewise for the second section for multiple declaration files.

回答8:

I used to have in the same project .c and .cpp files with the same filenames. The files were in folders all over the place and the solutions provided by others created a mess, and folder hell (in my case). Even Release builds would overwrite Debug builds!

A good (not perfect) solution would be to use $(ParentName), but for some reason beyond anyone's grasp it has been removed from later versions of Visual Studio (2015+).

What I use succesfully now is: $(IntDir)%(Filename)%(Extension).obj

which at least separates .c built object files from .cpp.

来源：https://stackoverflow.com/questions/3695174/visual-studio-2010s-strange-warning-lnk4042