[CMake] Feature request: generic (config-like) selector on target_link_libraries

[Denis Scherbakov]

> Specifying the lib4 dependency in the exe? But what about
> dependencies
> order in the link line? In my example, lib4-A or lib4-B
> must appear
> after lib3, if I want lib4 symbols required by lib3 to be
> added. In your
> solution, lib4 might as well be added by cmake just after
> lib1 and
> before lib3, which will result in a link failure.

IF( SOMETHING )
  SET (LIB4_VER "lib4a")
ELSE( SOMETHING )
  SET (LIB4_VER "lib4b")
ENDIF( SOMETHING )

ADD_EXECUTABLE(exe1 ${exe1_SRCS})
TARGET_LINK_LIBRARIES(exe1  lib1 lib2 lib3 ${LIB4_VER} lib5)

> More over, the executable is even not supposed to know that
> it depends
> on lib4. It just knows it depends on lib1, which knows it
> depends on
> lib3, which knows it depends on lib4. (I gave a simple

ADD_LIBRARY(lib1a ...)
TARGET_LINK_LIBRARIES(lib1a  lib2a)

ADD_LIBRARY(lib2a ...)
TARGET_LINK_LIBRARIES(lib2a  lib3a)

ADD_LIBRARY(lib3a ...)
TARGET_LINK_LIBRARIES(lib3a  lib4a)

ADD_LIBRARY(lib4a ...)
TARGET_LINK_LIBRARIES(lib4a  lib5)

# Etc

IF( SOMETHING )
  SET (LIB1_VER "lib1a")
ELSE( SOMETHING )
  SET (LIB1_VER "lib1b")
ENDIF( SOMETHING )

ADD_EXECUTABLE(exe1 ${exe1_SRCS})
TARGET_LINK_LIBRARIES(exe1  ${LIB1_VER})

> example, but the
> real project is a large one with many dependencies,
> including circular

CMake is not a chicken-and-egg solution. It is a build system.

> ones, and we don't want / can't easily know all
> libX-A/B that must be
> added to every executable)

In one CMake build tree you can have only one target LIB1, LIB2 and LIB3.
In your example LIB3 depends on LIB4a and at the same time - on LIB4b which
is not possible. Bool can be either TRUE or FALSE, but not both.

In your example (below)
exe1-A -> lib1 -> lib3 -> lib4-A -> lib5 -> lib7
exe1-B -> lib1 -> lib3 -> lib4-B -> lib6 -> lib8
you know that you need A or B only at link time. This means that you need
to specify ALL required dependencies at link time.

If you knew, what you need at LIB3 stage, then you would have done it already.

I see the following solutions:

1) LIB1A, LIB2A, etc
2) For each executable generate separate BUILD directory
   where there will be only one lib1->lib2->lib3->lib4->lib5 chain.
3) Specify what you want at link time.


> binaries: for some
> of them, lib3 must depend on lib4-A, and for some others,
> the same lib3
> must then depend on lib4-B.

It is not possible.

Denis