Avoiding many "../include" in CMake - c++

Question

I just started getting into CMake with C++ and was wondering how other programmers avoid having to do "../include" in all their CMakeFiles.txt.
One example is here: https://github.com/clab/cnn/blob/master/examples/CMakeLists.txt
They create an executable for each example without having to call INCLUDE_DIRECTORIES(...).
I tried adding the headers when calling ADD_LIBRARY(...), but that didn't seem to work.
Example:
tl/src/CMakeLists.txt:
SET(SRCS "x1.cpp" "x2.cpp")
SET(HDRS "../include/tl/x1.h" "../include/tl/x2.h")
INCLUDE_DIRECTORIES("../include")
ADD_LIBRARY(test_lib ${SRCS} ${HDRS})
tl/CMakeLists.txt:
PROJECT(TEST_LIB VERSION 0.1)
ADD_SUBDIRECTORY("src")
tl/examples/CMakeLists.txt:
INCLUDE_DIRECTORIES("../include")
ADD_EXECUTABLE(e1 e1.cpp)
TARGET_LINK_LIBRARIES(e1 test_lib)
Edit: I believe that INCLUDE_DIRECTORIES(...) is only necessary one per each directory throughout the tree.

Answer 1

Just add the INCLUDE_DIRECTORIES command at the top level. No need to explicitly add included files then.
tl/CMakeLists.txt:
PROJECT(TEST_LIB VERSION 0.1)
INCLUDE_DIRECTORIES("include")
ADD_SUBDIRECTORY("src")
ADD_SUBDIRECTORY("examples")
tl/src/CMakeLists.txt:
ADD_LIBRARY(test_lib "x1.cpp" "x2.cpp")
tl/examples/CMakeLists.txt:
ADD_EXECUTABLE(e1 e1.cpp)
TARGET_LINK_LIBRARIES(e1 test_lib)

Answer 2

How about this? tl/CMakeLists.txt
SET(SRCS "x1.cpp" "x2.cpp")
SET(HDRS "../include/tl/x1.h" "../include/tl/x2.h")
INCLUDE_DIRECTORIES("${CMAKE_PROJECT_DIR}/include")
ADD_LIBRARY(test_lib ${SRCS} ${HDRS})
You do not need to add headers to ADD_LIBRARY or ADD_EXECUTABLE if you want to compile your programs and libs. Only source files "cpp,c,cxx,..." are required because through the include macro #include you tell the compiler where it finds the header files.
With INCLUDE_DIRECTORIES(...) you only add a search path to the compiler where to look for headers. If you have your headers in the same directory as your source you don't need another search path. Also subpaths with include macro like this #include "../../include" is possible. So it really depends on the structure of your source files. Also remember, compiler settings setups know where some of the system headers are found. That is why you also do not need to define them.
And last but not least there are cmake scripts and pkg search files where adding paths to specific libraries is done automatically.

Answer 3

This is how I did it in a project of mine:
file(GLOB_RECURSE SRC
engine/*.cpp
platfoorm/*.cpp
)
file(GLOB_RECURSE INCLUDES
engine/*.h
platform/*.h
)
This generate an internal "hardcoded" list of files that the generated makefil will use, so if you add a new source file you will need to re-run cmake.
Of course you can change and add paths, the one in the example are the one I used in my project.