CMake library dependencies - c++

Question

This may be a tad hard to explain in full. I have setup a directory structure for a series of C++ libraries I am writing. I intend to use CMake to handle building these libraries. For the most part these libraries are completely separate "subsystems" but in the odd places one library needs access to the header file contained within another. Here is my directory structure.
base
├─ cmake
├─ docs
├─ examples
└─ sources
├─ libA
│ ├─ include
│ │ └─ libA
│ │ ├─ core.h
│ │ └─ *.h
│ ├─ source
│ │ └─*.cpp
└─ libB
├─ include
│ └─ libB
│ ├─ message.h
│ └─ *.h
└─ source
└─ *.cpp
There are CMakeLists.txt files in a few places. The main one is within the root directory base which sets up a bunch of variables and options and lastly calls add_subdirectory(sources). Within sources the CMakeLists.txt file simply calls:
add_subdirectory(libA)
add_subdirectory(libB)
Lastly there are CMakeLists.txt files in both the libA and libB directories. These are both largely the same except for the target name. An example of one of these files is:
set(target libA)
# Sources
set(include_path "${CMAKE_CURRENT_SOURCE_DIR}/include/${target}")
set(source_path "${CMAKE_CURRENT_SOURCE_DIR}/source")
# Add include directories
include_directories("${CMAKE_CURRENT_SOURCE_DIR}/include")
# Set the source files to compile
file(GLOB_RECURSE sources ${source_path}/**/*.cpp)
# Build library
add_library(${target} ${sources})
Now separately both of these libraries can be built without issue but I need to be able to include a header from libA in a source file within libB
// base/sources/libB/source/message.cpp
#include <ctime>
#include <iostream>
#include <libA/core.h>
#include <libB/message.h>
However when I build this I get the following error:
fatal error: 'libA/core.h' file not found
I have tried using target_link_libraries and target_include_directories without any luck. Clearly I am using these wrong or simply don't understand what each of these do.

Answer 1

Since your library libB needs a header from libA to compile, let's say libA is a dependency of libB. Fortunately, CMake is very clever at handling dependencies, so your job is pretty easy:
libA
Instead of
# Add include directories
include_directories("${CMAKE_CURRENT_SOURCE_DIR}/include")
add to the end
target_include_directories(${target} PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/include")
This will tell CMake that it must also include the headers for libA to the other projects that need it.
libB
Just tell CMake that libB needs libA:
target_link_libraries(${target} libA)
Note that this doesn't relate directly to the linking step when you compile. For libraries it mainly tells that there is this dependency, and in this case it will automatically add the paths for the headers automatically.

Answer 2

You can add to your CMakeLists.txt for libB, another include directory:
include_directories("${CMAKE_CURRENT_SOURCE_DIR}/../libA/include")