[cmake-developers] Additional properties for PCH support

[Peter Kuemmel]

> On 11/26/2013 12:32 PM, Peter Kuemmel wrote:
> > So, is this the right way to improve the pch situation?
> 
> Please summarize the situation here so we don't have to read the
> entire history of issue 1260 and all the proposed approaches.

As you already said, many attempts to solve the problem with
existing cmake functionality with no real "winner".

> 
> IIUC the modules in issue 1260 are all a workaround for lack for
> first-class support for PCH in CMake.  The right way forward is
> to add such support.  The main reason this has not happened is
> that no one has put in the time to figure out how to do it right.
> 
> There needs to be a CMake interface for specifying PCH information.
> Then the CMake C++-implemented generators can add the right rules.

I'm not sure if it is a good idea to implement the pch support
completely on the C++ side: There are so many generator/compiler/OS
combinations (>10*3*4), and when we restrict the solution to C++-only
we only have a couple of developers. But when we provide a mixed
solution (cmake extensions which could be used by a new cmake-script
module) then many of the cmake users will try to fix this module
when it is broken for their setup.

I think we should find a nice interface provided by a new module
which makes sense for all generators, and we should extend 
cmake's C++ only only to simplify the implementation of this module.

> Can you propose such an interface for discussion?

Because it is hard to "propose a interface" without any working
examples in the background I think we should start with my patch
and see if we could make a good solution for at least msvc's 
and gcc's non-IDEs generators.

> 
> I do not like the OBJECT_LOCATION property.  It ties too deeply
> with internal generator implementation details and has no meaning
> on Xcode or VS generators.  Whatever CMake interface is designed
> to configure PCH information can include a PCH-specific way of
> setting the location.

With this property we have similar solutions for msvc and gcc
which could be a starting point of a "official" pch macro.

But ATM I don't know if these properties are usable for the IDEs.

> 
> -Brad
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