[CMake] Recursively include target include directories only

[Dan Liew]

On 17 June 2015 at 12:28, Robert Dailey <rcdailey.lists at gmail.com> wrote:
> Is there a way to only take (recursively) the include directiories from
> a target or set of targets? I know that include directories propagate
> when passing targets to target_link_libraries(), but I do not want to
> link the libs; I only want the include directories.
>
> How can I do this? Thanks.

I presume recursively you mean propagating the required include
directories between targets rather anything to do recursively handling
directories.

As you mentioned include directories do propagate but this only
happens under certain conditions (when using PUBLIC or INTERFACE
scopes).

I don't know of a way of doing it without target_link_libraries but if
it is necessary for you to do this it sounds like you have bigger
problems relating to the way you have your project header files
organised.

The approach I use for include directories is to have all includes for
the project rooted in a single include directory in the source tree
and have all includes relative to that directory. Then if I you

```
include_directories(${CMAKE_SOURCE_DIR}/include)
include_directories(${CMAKE_BINARY_DIR}/include) # for generated header files
```

in the root of the project you never need to set include paths
anywhere else for sources that are in your project tree. E.g. if there
is header file in

${CMAKE_SOURCE_DIR}/include/project/moduleA/interface.h

then in the sources this is included as

#include "project/moduleA/interface.h"

(note there are never relative header file includes).

This is the approach that the LLVM project uses which is very simple
and very clean (take a look at the sources). This also works very well
for installing your header files, the contents of ${CMAKE_SOURCE_DIR}
just need to be copied into /usr/include .