C++ header files intermittently not found

Question

I have noticed some odd behaviour in VS2012 while working with some C++ projects. If I put the following line at the top of a header file: (myclass.h)

#include <D3DX11.h>

The compiler will tell me that it can't find the file (even though intellisense suggests it). However, if I put the same line in a different file (myclass.cpp) everything works just fine. I don't understand why it can be found when used in one file, but not the other? What is going on ?

EDIT / NOTE:
This isn't really a DirectX specific issue. That is just what I was working with when I decided to ask. This can happen in other places as well.

Answer 1

In the newer versions of visual studio the include paths are set at project-level. Maybe the project containing the cpp has the proper path set (see the comment from Jesse Good in your question). However that might not be the case for another project in your solution. As soon as you put the include in the header, all other projects including your header will need to know where to find the directX-headers as well.

Beware of intellisense - if you open a header it has to guess what cpp you might include this header from and the displayed info is not always correct. I think in the newer versions it even depends on what other files are opened / project is selected. The compiler should inform you what cpp actually causes problems when including the header.