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

Posted by: admin November 30, 2017 Leave a comment

Questions:

I have a C++ project where I’ve used directories as more of an organizational element — the way one might use packages in Java or directories in PHP. Directories are not intended to be self-sufficient elements, but rather just a way of organizing the whole of the project and keeping me from being overwhelmed by sources. How can I construct my CMakeLists.txt files to deal with this? Making the directories libraries doesn’t seem to fit here, since they are all interdependent and not intended to be used that way.

As a related issue, most of the examples I’ve seen of multiple subdirectories in CMake (and there aren’t very many of those) have ignored or glossed over the issue of setting include_directories, which is something I’ve been having trouble with. Short of combing my source files to determine which file depends on which and in what directory, is there anyway to just set all directories under /src/ as potential include directories and let CMake work out which ones are actually dependent?

Here’s an example structure:

--src
  --top1
    --mid1
      --bot1
        --src1.cpp
        --hdr1.h
      --bot2
        --src2.cpp
        --hdr2.h
    --mid2
      --bot3
        --src3.cpp
        --src4.cpp
        --hdr3.h
  --top2
    --mid3
      --src5.cpp
      --hdr4.h

So on and so forth. How can I structure my CMakeLists.txt files to handle this sort of structure?

Answers:

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 3.0)

project (Foo)

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

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.

Questions:
Answers:

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).