[CMake] Prefix header for Makefiles

[Jakob van Bethlehem]

Hej Andrew,

CMake does never scan source files (as far as I know), as it is not a compiler. From your question it almost seem you are making this assumption, so I just wanted to make sure to mention at least this.

Then, for your particular issue: two things that come to my mind:
* I’d imagine you have somewhere a add_executable(main Main.cpp Prefix.h DependHeader.h) - as far as I can tell, the only thing that really matters, is that the main executable gets recompiled whenever you make a change in any of the headers. To put it differently, I *don’t* think you (should) need the OBJECT_DEPENDS.
* If you’re worried about scaling, maybe you could have a look at the file(GLOB ) command. However, be sure to also carefully read the potential problems with that approach. In my current (big) project, we explicitly list each source file for most parts of the source tree. In practice you’re surely *not* going to add all 50 new files at once. It is a very simple thing to add the newly created file to some variable in the correct CMakeLists.txt file. You just have to remember to do it, it’s really not a big deal.

By the way, we’re actually using the PrecompiledHeader script - it works flawlessly.

Sincerely, Jakob 

> On 26 Aug 2015, at 11:48, Andrew Shaitorov <andrew.shaitorov at gmail.com> wrote:
> 
> Hi All!
> 
> I have a very annoying problem with CMake Makefiles generator.
> 
> I'm using prefix header (see attached archive for minimized test).
> 
> There is one source file named Main.cpp and two header files: Prefix.h and DependHeader.h. Prefix.h includes DependHeader.h and Main.cpp includes Prefix.h by using prefix header compiler flag (-include for GCC/Clang and /FI for MSVC):
> 
> 	set_source_files_properties(Main.cpp PROPERTIES
> 		COMPILE_FLAGS "-include Prefix.h"
> 		OBJECT_DEPENDS ${CMAKE_CURRENT_LIST_DIR}/Prefix.h)
> 
> When I change Prefix.h, then make recompiles Main.cpp as needed. But when I make a change in DependHeader.h, then no recompilation happens. Its clear for me that I have to specify all depend files in OBJECT_DEPENDS option, but in real project there may be a lot of depend files with nested includes. And the most annoying thing is that the project compiles without any errors and may just crash because of struct layout or other critical changes in depend headers.
> 
> I seems like the following script have the same problem when using FORCE_INCLUDE option:
> 
> https://github.com/larsch/cmake-precompiled-header/blob/master/PrecompiledHeader.cmake <https://github.com/larsch/cmake-precompiled-header/blob/master/PrecompiledHeader.cmake>
> 
> I looked in CMake source code and didn't find a way to add Prefix.h in a list for dependency scanning. Also I don't know how to run depend scanner on Prefix.h in CMake script to generate a list of all depend files for OBJECT_DEPENDS option.
> 
> Any advise?
> 
> Best,
> Andrew.
> 
> <CMakePrefixHeaderTest.zip>-- 
> 
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