how to use cmake to compile modern c++ project code

Question

here's my source code directory structure.

some project
    libs
        mylib1
            ...
        3rdlibs1
            ...
    apps
        myapp1
            ...

My requirments are as follows:

• 3rdlibs1 should be compiled when using command. like "cmake -G ...".
• 3rdlibs1 should be compiled as static library.
• mylib1 should be modular.
• mylib1 depends on 3rdlibs.
• myapp depends on mylib1, and it should only link to mylib1. It shouldn't depends on 3rdlibs or system libraries.

Can you give me some sample code. I know ExternalProject_Add can help me to compile 3rdlibs. But I really don't know how to do it. I think other people may also be interesting to this question.

Answer 1

If you have all your sources in single file system tree, it is better to use add_subdirectory than ExternalProject . ExternalProject is for projects which are truly external, eg on remote server or VCS repository. Unlike add_subdirectory which can create target of any type, ExternalProject can create only UTILITY target, similar to add_custom_target command. UTILITY targets have limitations, eg you can not use them in target_link_libraries command. Using add_subdirectory is much simpler. Top-level:

cmake_minimum_required(VERSION 3.7)
project("some_project")

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_subdirectory(libs/3rdlibs1)
add_subdirectory(libs/mylib1)
add_subdirectory(apps/myapp1)

App:

project("myapp1")

set(SRC_FILES ...)
add_executable(myapp1 ${SRC_FILES})
target_link_libraries(myapp1 PRIVATE mylib1)

Lib. As I understand from your description, 3rdlibs1 is optional dependency of mylib1. Otherwise, how can myapp use mylib1 without 3rdlibs1?

project("mylib1")

option(THIRD_LIBS_SUPPORT "description" OFF)

set(SRC_FILES ...)
add_library(mylib1 STATIC ${SRC_FILES})
#PUBLIC means that both mylib1 and its dependents use the headers
target_include_directories(mylib1 PUBLIC "${PROJECT_SOURCE_DIR}/include")

if(THIRD_LIBS_SUPPORT)
    #PUBLIC means that 3rdlibs1 will be linked to mylib1 dependents
    target_link_libraries(mylib1 PUBLIC 3rdlibs1)
    target_compile_definitions(mylib1 PUBLIC -DTHIRD_LIBS_SUPPORT)
endif()