CMake with multiple sub projects building into one directory - c++

Question

I'm not very familiar with CMake and still find it quite confusing. I have a project that has a server and client that I want to be able to run independent of each other but that builds together into the same directory (specifically the top level project build directory kind of like how games have the server launcher and game launcher in the same directory) Currently it just creates a builds directory in each sub project, so one in client, one in server etc.
This is my current project structure
.
├── CMakeLists.txt
├── builds
│   ├── debug
│   └── release
├── client
│   ├── CMakeLists.txt
│   ├── assets
│   └── source
│   └── Main.cpp
├── documentation
├── libraries
│   ├── glfw-3.3.7
│   └── glm
├── server
│   ├── CMakeLists.txt
│   └── source
│   └── Main.cpp
└── shared
├── PlatformDetection.h
├── Utility.h
├── events
└── platform
├── linux
├── macos
└── windows
This is my root CMake file
cmake_minimum_required(VERSION 3.20)
project(Game VERSION 1.0.0)
add_subdirectory(libraries/glfw-3.3.7)
add_subdirectory(client)
add_subdirectory(server)
Client CMake file
cmake_minimum_required(VERSION 3.20)
project(Launcher LANGUAGES CXX VERSION 1.0.0)
set(CMAKE_CXX_STANDARD 23)
set(SOURCE_FILES source/Main.cpp ../shared/events/Event.h ../shared/Utility.h
source/Client.cpp source/Client.h ../shared/PlatformDetection.h ../shared/events/EventManagementSystem.cpp
../shared/events/EventManagementSystem.h)
set(GLFW_BUILD_DOCS OFF CACHE BOOL "" FORCE)
set(GLFW_BUILD_TESTS OFF CACHE BOOL "" FORCE)
set(GLFW_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
include_directories(${CMAKE_SOURCE_DIR}/libraries/glm)
include_directories(${CMAKE_SOURCE_DIR}/libraries/glfw-3.3.7/include/GLFW)
include_directories(${CMAKE_SOURCE_DIR}/shared)
add_executable(Launcher ${SOURCE_FILES})
target_link_libraries(Launcher LINK_PUBLIC glfw)
Server CMake file
cmake_minimum_required(VERSION 3.20)
project(ServerLauncher LANGUAGES CXX VERSION 1.0.0)
set(CMAKE_CXX_STANDARD 23)
set(SOURCE_FILES source/Main.cpp ../shared/events/Event.h ../shared/Utility.h
../shared/PlatformDetection.h ../shared/events/EventManagementSystem.cpp
../shared/events/EventManagementSystem.h)
include_directories(${CMAKE_SOURCE_DIR}/libraries/glm)
include_directories(${CMAKE_SOURCE_DIR}/shared)
add_executable(ServerLauncher ${SOURCE_FILES})
How can I make the client and server build into the same directory? And can these cmake file structures be improved at all? They seem quite messy and all over the place to me though that may just be due to my unfamiliarity with CMake.

Answer 1

You cannot have multiple subdirectories use the same build directory, but that doesn't seem what you're trying to achieve.
Assuming you don't set the variable CMAKE_RUNTIME_OUTPUT_DIRECTORY anywhere in your project, and you don't specify the RUNTIME_OUTPUT_DIRECTORY target property for any of your targets by some other means, you could simply set the variable in the toplevel CMakeLists.txt before using add_subdirectory:
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/bin)
add_subdirectory(...)
...
Note that for distributing the program you should be using install() logic:
Client CMakeLists.txt
...
install(TARGETS Launcher RUNTIME)
Server CMakeLists.txt
...
install(TARGETS ServerLauncher RUNTIME)
Note that you may need to add logic for installing dependencies.
Using those install commands allows you to use
cmake --install <build dir> --prefix <install dir>
to install the programs locally in the default directory for binaries on the system. Furthermore it's the basis for packaging your project using cpack.