[CMake] Confusion over INTERFACE vs STATIC|SHARED IMPORTED vs INTERFACE IMPORTED

[Nicholas Braden]

There is one difference that I thought of immediately, and I tested it
- an INTERFACE target can be installed/exported, but an INTERFACE
IMPORTED target cannot be installed/exported because CMake considers
it to not be in the build/source directory. I'm sure there are other
internal differences due to the fact that imported targets aren't
considered to be inside the build or source directories.

I think the main takeaway is that "INTERFACE IMPORTED" is not actually
a different kind of target, it is just a target which is both
INTERFACE and IMPORTED. Why not allow all four combinations of
INTERFACE and IMPORTED?

On Tue, Jul 5, 2016 at 7:35 PM, Ted Middleton <middleton.ted at gmail.com> wrote:
> Ack - sorry. s/INTERFACE_LINK_LIBRARIES/IMPORTED_LOCATION/cg.
> INTERFACE_LINK_LIBRARIES I guess would be a completely valid property to set
> on an add_library(foo INTERFACE) target. But IMPORTED_LOCATION wouldn't,
> right?
>
> "INTERFACE IMPORTED would then mean there is no compiled
> binary to be linked against, just as INTERFACE alone means there are
> no sources to create said binary from"
>
> Is there a difference between them, then? If neither produce an actual build
> rule, and neither can point to a .lib/.so/.dll/.a/.dylib, are they not
> exactly the same thing? The documentation says that add_library(foo
> INTERFACE IMPORTED) creates an /imported target/, but the explanation of
> what exactly this is is ... confusing? Based on the description of what an
> /imported target/ is, I would think that it is the same thing that
> add_library(foo <STATIC|SHARED> IMPORTED) produces?
>
>
>
> On Tue, Jul 5, 2016 at 4:09 PM, Nicholas Braden <nicholas11braden at gmail.com>
> wrote:
>>
>> From what I understand, interface libraries just don't have sources or
>> build results. INTERFACE IMPORTED would then mean there is no compiled
>> binary to be linked against, just as INTERFACE alone means there are
>> no sources to create said binary from. At least, that is my
>> understanding.
>>
>> > My understanding of this is that setting INTERFACE_LINK_LIBRARIES on a
>> > target like this would be an error?
>>
>> Why can't a header-only library link to non-header-only libraries?
>>
>> On Tue, Jul 5, 2016 at 5:17 PM, Ted Middleton <middleton.ted at gmail.com>
>> wrote:
>> > I'm looking at the documentation for add_library(), and I'm really
>> > confused
>> > about the distinction between add_library(foo INTERFACE) vs
>> > add_library(foo
>> > INTERFACE IMPORTED) vs add_library(foo <SHARED|STATIC> IMPORTED).
>> >
>> > Correct me if I'm wrong (and I probably am), but add_library(foo
>> > INTERFACE)
>> > is for propagating properties like INTERFACE_INCLUDE_DIRECTORIES to
>> > other
>> > targets using target_link_libraries(), so that header-only libraries can
>> > be
>> > consumed by other targets conveniently. My understanding of this is that
>> > setting INTERFACE_LINK_LIBRARIES on a target like this would be an
>> > error?
>> >
>> > And add_library(foo SHARED IMPORTED) would be to create a target object
>> > for
>> > a pre-existing library (like a platform lib or something) for which
>> > cmake is
>> > not supposed to try to create an actual "call the compiler and
>> > linkeditor or
>> > archiver" build rule. This still boils down to mostly propagating
>> > properties, no? Except now some of those properties can be things like
>> > INTERFACE_LINK_LIBRARIES?
>> >
>> > So can anyone tell me what add_library(foo INTERFACE IMPORTED) is? The
>> > documentation says,
>> >
>> > "An INTERFACE Imported Target may also be created with this signature.
>> > An
>> > IMPORTED library target references a library defined outside the
>> > project.
>> > The target name has scope in the directory in which it is created and
>> > below,
>> > but the GLOBAL option extends visibility. It may be referenced like any
>> > target built within the project. IMPORTED libraries are useful for
>> > convenient reference from commands like target_link_libraries()."
>> >
>> > That description is what I thought add_library(foo <SHARED|STATIC>
>> > IMPORTED)
>> > was for?
>> >
>> >
>> >
>> > --
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