How can I build a C++ project with multiple interdependent subdirectories?

Since the directory structure in your project is just there to keep your files organized, one approach is to have a CMakeLists.txt that automatically finds all sources files in the src directory and also adds all directories as include directories that have a header file in them. The following CMake file may serve as a starting point:

cmake_minimum_required(VERSION 3.12)

project (Foo)

file (GLOB_RECURSE Foo_SOURCES CONFIGURE_DEPENDS "src/*.cpp")
file (GLOB_RECURSE Foo_HEADERS CONFIGURE_DEPENDS "src/*.h")

set (Foo_INCLUDE_DIRS "")
foreach (_headerFile ${Foo_HEADERS})
    get_filename_component(_dir ${_headerFile} PATH)
    list (APPEND Foo_INCLUDE_DIRS ${_dir})
endforeach()
list (REMOVE_DUPLICATES Foo_INCLUDE_DIRS)

add_executable(FooExe ${Foo_SOURCES})
target_include_directories(FooExe PRIVATE ${Foo_INCLUDE_DIRS})

The two file(GLOB_RECURSE ... commands determine the set of source and header files. The foreach loop computes the set of include directories from the list of all header files. The CONFIGURE_DEPENDS flags tells CMake to re-run the glob command at build time.

One drawback with computing the set of source files is that CMake will not automatically detect when new files are added to your source tree. You manually have to re-create your build files then.

Though @sakra gave a good answer to this question, I believe it is more proper to approach it more in depth.

We want to separate our code into modules and libraries for many reasons. Like code encapsulation, re usability, easier debugging etc. This idea would propagate in compiling process too.

In other word, we want to divide the compilation process into little compilation steps, each belong to one module. So every module must have its own compilation procedure. This is why we use one CMakeLists.txt file per directory. Hence every directory would have its own compilation commands and there would be one master CMakeLists.txt file in the root directory of your project.

Here is an example. Consider the following structure of a project:

src/
|
- main.cpp
|
_sum/
    |
    - sum.h
    |
    - sum.cpp

We would have one CmakeLists.txt Per directory. First directory is the root directory of the project which src/ folder is in it. here is content for that file:

cmake_minimum_required(VERSION 3.4)
project(multi_file)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_FLAGS "-Wall")

add_subdirectory(src) 

Next CMakeLists.txt would located in src/ directory:

add_subdirectory("sum")

add_executable(out main.cpp)
target_link_libraries(out sum)

And the last one will be in the sum/ directory:

add_library(sum SHARED sum.cpp)

I hope this helps. I created a github repository in case you feel you need to see the code or you need further explanation.

I'm not an expert on CMake but since there are no other answers I'll take a look at the documentaton and give it a go. Organizing source and include files in different directories is pretty much the norm.

It looks like CMake allows you to give a list of include directories: http://www.cmake.org/cmake/help/cmake-2-8-docs.html#command:include_directories

So something like:

include_directories("src/top1/mid1/bot1" "src/top1/mid1/bot2/" ... )

These are passed to the compiler so it can find the header files and will be passed for each of the source files. So any of your source files should be able to include any of the header files (which I think is what you're asking for).

Similar to that you should be able to list all your source files in the add_executable command:

add_executable(name "src/top1/mid1/bot1/src1.cpp" "src/top1/id1/bot2/src2.cpp" ...)

So this would be a naive way of getting everything to build. Each source file will be compiled and will look for headers in all those directories and then the object files will get linked together. Consider if there is any way of simplifying this such that you don't need so many include folders, maybe there are only a few common header files that need to be referenced by all source files. If things get more complex you can buiild sub-hierarchies into libraries etc. Also consider seperating source files and headers (e.g. in src and include).