Visual Studio 2010's strange "warning LNK4042"

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.

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.

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.