c++ - When do i want to turn off "precompiled header" in visual studio?

Question

First of all i want to say that I read about precompiled headers and I understand that this is an optimization that saves me the time of compiling headers over and over on every built.

I'm reading the documentation of boost and I see that in the instructions they say:

In Configuration Properties > C/C++ > Precompiled Headers, change Use Precompiled Header (/Yu) to Not Using Precompiled Headers

And then they explain it:

There's no problem using Boost with precompiled headers; these instructions merely avoid precompiled headers because it would require Visual Studio-specific changes to the source code used in the examples.

1. Can some explain me the sentence I marked in bold? which visual studio specific changes they are talking about ? (Here is the link to the documentation I'm reading: http://www.boost.org/doc/libs/1_55_0/more/getting_started/windows.html#pch)
2. Why and when I would want to turn off the precompiled headers?
3. what is the difference between "Create" and "Use" in the precompiled header options.

Answer 1

Originally a comment, but I may as well post it. Note: this is specific to VC++:

1. The bold sentence is their way of saying the samples don't follow the mantra of a unified use-this-lead-in-header-for-pch-generation model. IOW, their samples aren't PCH-friendly, but you can still use pch with boost in your projects if properly configured.

2. You would turn them off for a variety of reasons. Some source modules, particularly ones from 3rd-parties, don't follow the PCH model of including "the" pch-through-header at their outset. Their samples are such code (and thus the advise to turn them off for their samples). Sometimes source files require different preprocessor configurations only for this files and not all files int he project; another reason to disable PCH for those files.

3. You typically use a source/header pair to generate "the One"; the precompiled header image. This header file typically includes:

   1. Any system standard lib headers used by your project
   2. 3rd-party SDK headers
   3. Just about everything else that is NOT in active development for your project.

The single source file tagged as Create typically includes one line of code : #include "YourHeaderFile.h", where YourHeaderFile.h is the header you filled with stuff from the list above. Tagging it as "Create" through header YourHeaderFile.h tells VC it is the file needed for rebuilding the PCH through that header when compiling other source files. All other source files are tagged as Use (except the ones where PCH is turned off) and should include, as their first line of code, the same #include "TheHeaderFile.h".

In short (hard to believe), <boost> is telling you their samples aren't setup like described above, and as such you should turn PCH off when building them.