CMake - Module + Library confusion

Question

I've started a new c++ project , and I am confused with all the CMake capabilities. I have tried to understand better by looking at examples and CMake tutorials

I should create a new project composed of:

1. Library: It contains some common classes that will be used by the following module(s) (e.g., vector, matrix, image, etc..)

2. Module (possibly more than 1 in the future): It contains some module-specific classes (e.g., classifier, estimator, etc.) and a main.

My proposed folder structure is as below:

|-- Root Project
    |-- CMakeLists.txt
    |
    |-- Library
    |   |-- CMakeLists.txt
    |   |-- include
    |   |   |-- CMakeLists.txt (?)
    |   |   `-- Lib_Class.h
    |   `-- src
    |       |-- CMakeLists.txt (?)
    |       `-- Lib_Class.h
    |
    |-- Application 1
    |   |-- CMakeLists.txt
    |   |-- include
    |   |   |-- CMakeLists.txt (?)
    |   |   `-- Method.h
    |   `-- src
    |       |-- CMakeLists.txt (?)
    |       |-- Method.cpp
    |       `-- main.cpp
    |
    |-- Application 2
    |   |-- CMakeLists.txt
    |   |
    `

The problem arises when I have to actually add the code to the different CMakeLists.txt files. According to my reasoning, I would have:

• Root/CMakeLists.txt: For creating the project and adding the subdirectories of the Library and the Module(s).
• Library/CMakeLists.txt: This creates the library with the header (from include folder) and source (from src folder) files.
• Module/CMakeLists.txt: This creates an executable from the src/main.cpp file using the Library and the module-specific classes with header files in include folder and source files in src folder.

I have 2 questions:

First, I also found solutions in other replies with CMakeLists.txt files in the Library/src and Module/src folders. But I really don't understand how to use them and what to write inside them, because I would have used only the CMakeLists.txt file in the parent folder.

Second, in case I want to link an external library (e.g., OpenCV or dlib) should I link it in the modules and library, individually, or should I link it in the root CMakeLists.txt file (provided that the library is used everywhere)?

I really need some assistance to try to understand CMAKE. Can someone explain or please direct me to a suitable tutorial on this subject.

---

Matthieu, thank you very much for your help. According to the explanation you provided me, I came out with the following CMakeLists.txt files:

Root/CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(Project_Name)

add_subdirectory(Library)
add_subdirectory(Application)

Library/CMakeLists.txt

project(Library)

set(LIB_HEADERS 
    include/Lib_Class.h
)

set(LIB_SOURCES 
    src/Lib_Class.cpp
)

add_library(Library_Name SHARED ${LIB_SOURCES} ${LIB_HEADERS})

Application/CMakeLists.txt

project(Application)

set(APP_HEADERS 
    include/Method.h
)

set(APP_SOURCES 
    src/Method.cpp 
    src/main.cpp
)

add_executable(Application_Name ${APP_SOURCES} ${APP_HEADERS})
target_link_libraries(Application_Name Library_Name)

Now everything seems to work grat! Thank you again and sorry again for being confusing somethimes!

Answer 1

The root cmakelists should set up all the variables, checking compiler support and library presence.

Then you go to each subfolder and create the libraries and executables based on the source code and the detected libraries. You should also set up all the linked libraries there.

Then cmake will figure out what depends on what.