Linker Tools Warning LNK4221

This object file does not define any previously undefined public symbols, so it will not be used by any link operation that consumes this library

Consider the following two code snippets, a.cpp:

// a.cpp
#include <atlbase.h>

And b.cpp:

// b.cpp
#include <atlbase.h>
int function()
{
   return 0;
}

To compile the files and create two object files, run cl /c a.cpp b.cpp at a command prompt. If you link the object files by running link /lib /out:test.lib a.obj b.obj, you'll receive the LNK4221 warning. If you link the objects by running link /lib /out:test.lib b.obj a.obj, you won't receive a warning.

No warning is issued in the second scenario because the linker operates in a last-in first-out (LIFO) manner. In the first scenario, b.obj is processed before a.obj, and a.obj has no new symbols to add. By instructing the linker to process a.obj first, you can avoid the warning.

A common cause of this error is when two source files specify the option /Yc (Create Precompiled Header File) with the same header file name specified in the Precompiled Header field. A common cause of this problem deals with pch.h since, by default, pch.cpp includes pch.h and doesn't add any new symbols. If another source file includes pch.h with /Yc and the associated .obj file is processed before pch.obj, the linker will throw LNK4221.

One way to resolve this problem is to make sure that for each precompiled header, there's only one source file that includes it with /Yc. All other source files must use precompiled headers. For more information about how to change this setting, see /Yu (Use Precompiled Header File).