[CMake] Alias of imported target

[Ruslan Baratov]

On 05-May-15 20:01, Stephen Kelly wrote:
> CHEVRIER, Marc wrote:
>
>> Hi,
>>
>> Currently it is not possible to create an alias target for an imported
>> target (I.e. add_library with ALIAS keyword). But I don’t understand what
>> is the technical constraint or semantic reason behind this limitation. If
>> anybody can explain this limitation, it will be nice.
> It was designed with as many restrictions as I could think of (everything
> which was not part of the need to add it at all):
>
>   http://www.cmake.org/gitweb?p=cmake.git;a=commitdiff;h=370bf554
>
> The restrictions are there because they are easy to lift later when a need
> arises without breaking user code. It also means that design considerations
> which were not known or understood two years ago can be considered in
> designing new interfaces.
>
> Do you have a need for ALIAS IMPORTED libraries? Can you describe it?
>
> Thanks,
>
> Steve.
>
>
It may be helpful in situations when you build package with examples. In 
case when you want examples to work both as a part of a project 
(add_subdirectory) or stand-alone (find_package).

For instance:
     if(TARGET bar)
       # bar and boo targets in the same project
       add_library(Foo::bar ALIAS bar)
     else()
       # boo installed first, bar build as a stand-alone project
       find_package(Foo CONFIG REQUIRED) # target Foo::bar
     endif()
     target_link_libraries(boo Foo::bar)

If ALIAS will be allowed for imported targets we can write:

   if(NOT TARGET bar)
     find_package(Foo CONFIG REQUIRED)
     add_libarary(bar ALIAS Foo::bar)
   endif()
   target_link_libraries(boo bar)

We are not saving any space here and it's not critical feature, but this 
symmetry looks reasonable.

Ruslo