What's the most common header for visual c++?

if u are talking about common input and output then use stdio.h better buy any book and study from it

Code originally authored by Visual C++ tends to #include "stdafx.h" in each cpp file, although each project will have its own stdafx.h.

stdafx.h is a special file that is precompiled. A C++ application may include many kilobytes of library source that may take significant time for the compiler to chew through. Precompiled headers allow the compiler to chew through the stdafx.h file once, and then start from that known point every time it compiles a subsequent source.

There is nothing special about the stdafx.h name, you can change it if you wish, it's just the name Visual Studio dumps in projects by default.

All the other header files you're likely to encounter in a Visual C++ application will be dependant on what your targetting. However, given (a) Visual C++ runs on Windows and (b) 95% of computers run Windows, then windows.h is probably a common #include (directly or indirectly).

Many programs may rely on the standard C library, or C++'s STL, so although main header files are strong candidates, I would discount them as being niches.

Summary: #include "stdafx.h" is the most common #include line, but windows.h is the most included actual file.