[CMake] CMake and precompiled headers

[Brad King]

Gregor Berginc wrote:
> Hi all,
> 
> I noticed a feature request for precompiled headers (bug# 1260). One
> mentioned there, that you can simply use SET_SOURCE_FILES_PROPERTIES
> for the time being. But, unfortunatelly, I can't seem to get it
> working. I put my header to the list of sources:
> 
> SET(VC_SRC
>   includes.hh
>   ...
> )
> 
> Then I added the SET_SOURCE_FILES_PROPERTIES like:
> 
> SET_SOURCE_FILES_PROPERTIES(includes.hh COMPILE_FLAGS -c)
> 
> When I rerun cmake, I get the following error:
> 
> CMake Error: Source file
> "/home/lemmy/research/VisualCortex/src/includes.hh" has unknown type.
> 
> (The error code is 255).
> 
> Ok, so it seems CMake does not support my prefered header extension. I
> changed it to .hpp and cmake did not complain. But it also did compile
> my header file. I am using GNU C++ compiler.

Do you mean "did not compile my header file"?

Anyway, precompiled headers don't work that way.  You cannot just 
compile them to object files.  You have to have a compiler that supports 
precompiled headers.  Then read the documentation of that compiler to 
find the flag to enable precompiled header support.  A precompiled 
header is more of a "preparsed" header.  The compiler just stores a file 
somewhere that lets it load the pre-parsed header quickly.

-Brad