__w64 is a special key word of Visual C++ compiler by which we should mark the types which will become 64-bit when compiling the code on a 64-bit system. This allows Visual C++ compiler to perform some preliminary code tests for compatibility with 64-bit systems while it is built only in 32-bit mode. This diagnosis is performed if the key Wp64 is defined (Detect 64-Bit Portability Issues).
- Andrey Karpov. 64 bits, Wp64, Visual Studio 2008, Viva64 and all the rest...
- MSDN. Microsoft Specific keyword __w64.