[cmake-developers] New OBJECT library feature

[Brad King]

On 3/19/2012 2:25 PM, Eric Noulard wrote:
> 2012/3/19 Brad King<brad.king at kitware.com>:
>> Can you enumerate use cases when the order of objects matters?  Unlike
>> static libraries *all* objects will be included.  If there are duplicate
>> symbols it is an error.  If there are not duplicate symbols then how
>> does order matter?
>
> The custom command likn step looks like:
> COMMAND ${CMAKE_LINKER} --oformat binary -Ttext ${BM_IMAGE_ADDRESS}
> -melf_i386 -e _start -o ${BM_APPNAME}.bin ${BM_STARTUPOBJ}
> --start-group $<TARGET_LINKER_FILE:system>  $<TARGET_LINKER_FILE:xc>
> $<TARGET_LINKER_FILE:${BM_APPNAME}>  --end-group ${BM_LDSCRIPT}
>
> ${BM_STARTUPOBJ} is the specific object file compiled on its own using
> custom command.

I can imagine some cross compiling toolchains may want the object with
the entry point to come first.

> Now that I think about it, before that I was forced to create a custom
> command in order
> to compile the file ('with the same option as the one compiled by add_library)
> but now I will be able to build an "OBJECT library" containing a single file
> and refer to it using $<TARGET_OBJECTS:objlib>  which is perfectly fine for
> ensuring the ordering I may need for the link step.

Note that $<TARGET_OBJECTS:...> is not a full generator expression that
can be used from add_custom_command.  That is also reserved for the
future (which is why I chose that syntax).  Currently it is just a
special source file syntax recognized by add_library and add_executable.

-Brad