[cmake-developers] Issuing errors for faulty INTERFACE_INCLUDE_DIRECTORIES

[Alexander Neundorf]

On Wednesday 27 March 2013, Brad King wrote:
> On 03/26/2013 04:07 PM, Alexander Neundorf wrote:
> > there is a difference here in that the include directory properties are a
> > new feature, so this wouldn't break existing code.
> > It would force developers who make use of the new feature to split
> > exported targets into individual components and support components
> > properly in their Config.cmake files.
> 
> I don't want to force anyone to do that.  It is perfectly reasonable
> to have a monolithic targets file.

From readme.txt:
"A package can provide sub-components.
Those components can be listed after the COMPONENTS (or REQUIRED)
or OPTIONAL_COMPONENTS keywords.  The set of all listed components will be
specified in a Xxx_FIND_COMPONENTS variable.
For each package-specific component, say Yyy, a variable Xxx_FIND_REQUIRED_Yyy
will be set to true if it listed after COMPONENTS and it will be set to false
if it was listed after OPTIONAL_COMPONENTS.
Using those variables a FindXxx.cmake module and also a XxxConfig.cmake 
package configuration file can determine whether and which components have 
been requested, and whether they were requested as required or as optional.
For each of the requested components a Xxx_Yyy_FOUND variable should be set
accordingly.
The per-package Xxx_FOUND variable should be only set to true if all requested
required components have been found. A missing optional component should not
keep the Xxx_FOUND variable from being set to true."

We should try to make that possible.

find_package(Foo REQUIRED COMPONENTS Blub)

If this does not error out, and if the result of that is Foo_FOUND = TRUE and 
Foo_Blub_FOUND = TRUE, a cmake user should be able to expect that component 
Blub of Foo works and can be used.

Maybe instead of generating code which immediately results in failure at the 
find_package() step, how about generating code which contains a list of 
imported targets with missing dependencies/files/directories ?
This can then be evaluated in the Config.cmake file and the per-component 
_FOUND variables can be set.
Does otherwise the author of a Config.cmake file have to write cmake code to 
query the imported targets for their include interface properties, and check  
whether the referenced directories and targets exist ?

With such a variable, the code in a Config.cmake file could look like this:

foreach(comp ${Xxx_FIND_COMPONENTS})
   include(${CMAKE_CURRENT_LIST_DIR}/${comp}Targets.cmake OPTIONAL
           RESULT_VARIABLE fileIncluded)

   set(Xxx_${comp}_FOUND TRUE)
   if(NOT fileIncluded)
      set(Xxx_${comp}_FOUND FALSE)
   endif()

   # now the suggested variable:
   if(Xxx_IMPORTED_TARGETS_WITH_MISSING_STUFF)
      set(Xxx_${comp}_FOUND FALSE)
   endif()
endforeach()


Alex