What is the difference between “Create Precompiled Header” (/Yc) and “Use Precompiled Header” (/Yu) in Visual Studio?

Question

I read the documentation in MSDN, but in the end I didn\'t get a clear idea what is the practical difference between them, exactly. Both seem to require stdafx.h to be added

Answer 1

Short summary of how to use PCH files in Visual Studio:

• All cpp files in the project have to include stdafx.h (you can change this to something else if you wish)
• Select project in Solution Explorer and in Properties -> C++ -> Precompiled Headers set 'Create/Use precompiled headers' to 'Use'. Hit Apply
• While the Property Pages are still shown select stdafx.cpp in solution explorer and set the value to 'Create'

Answer 2

Well, I think that you must first understand the purpose of precompiled headers. In large projects, it can take ages to process all the headers required by a single client extension for example, so some people prefer to distribute the .pch files along with their libraries. In order to generate the .pch files, you use the /Yc compiler flag and the person who wants to consume your library will set the /Yu flag. See here and here for details.