[CMake] Precompiled headers (was Re: problem: using options other than -D with VS.NET)

[Brad King]

Erik van der Wal wrote:
> Brad King wrote:
> 
>> Bojan Resnik wrote:
>>
>>>    A related question - are there any plans to explicitly support
>>> generation and usage of precompiled headers for compilers that support
>>> it?
>>
>>
>>
>> Currently it is not on the feature list, but it sounds like a good 
>> idea.  Please add it to the bug tracker as a feature request:
> 
> 
> Well noticed, Bojan, I was indeed referring to precompiled headers. In 
> the wxArt2D library we currently use these (not implemented by me).
> Like Brad, I think cmake supporting precompiled headers is a good idea.
> The precompiled headers in wxart2d are implemented, by creating it using 
> some dummy.cpp file, which only includes the (central) header. This is 
> done by using the /Yc option.
> Next all following c++ files should use this file (/Yu option).
> This approach allways worked with VC6, but now we experience problems 
> with .NET.
> The first problem started this thread and was cmake not accepting the 
> /Yu option.
> The next problem is related to the compile order. The approach that was 
> taken, needs the precompiled header to be created first, i.e. compile 
> dummy.cpp first. Most generators I have seen so far, compile in the 
> order in which they have been put in the CmakeLists.txt. The .NET 
> generator however seems to compile in reverse order.
> 
> Questions:
> Is the compile order specified in cmake?
> If yes, is this a .NET generator bug?

I think the reverse ordering might be a bug, but I'm not sure.  Please 
add it to the bug tracker here:

http://www.cmake.org/Bug

so that it will at least be investigated at some point.

> If no, can ADD_DEPENDENCIES be used somehow, to first compile dummy.cpp?

Yes.  Create an executable target for dummy.cpp:

ADD_EXECUTABLE(precompile_headers dummy.cpp)
ADD_LIBRARY(mylib ...)
ADD_DEPENDENCIES(mylib precompile_headers)

This should make sure that the executable is built before the library.

-Brad