CMake with usual project structure

The error is:"CMake Error at src/CMakeLists.txt:2 (target_include_directories):Cannot specify include directories for target "hello" which is not built by this project."

The project structure is:

-build
-src
    -CMakeLists.txt
    -include
        -util
            -util.h
    -util
        -CMakeLists.txt
        -util.cpp
    main.cpp
CMakeLists.txt

I want to build the target hello in build, and here's my code(incomplete):

#project/CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

project(hello VERSION 1.0)

add_subdirectory(src)

#project/src/CMakeLists.txt
add_subdirectory(util)
target_include_directories(hello PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)

add_executable(hello main.cpp)

target_link_libraries(hello PUBLIC util)

#project/src/util/CMakeLists.txt
add_library(util STATIC util.cpp)

I'm new in CMake and I wonder what should I write in these 3 CMakeLists.txt. I will be thankful if anyone can tell me!

The error described in the comments results from target hello being accessed before it was created. The add_* family of commands creates targets while the target_* family of commands modifies existing targets. Create first, then do something with it.

In this case, we can clean up some more. The file project/src/CMakeLists.txt should look like this:

add_subdirectory(util)

add_executable(hello main.cpp)

target_link_libraries(hello PUBLIC util)

And project/src/util/CMakeLists.txt like this:

add_library(util STATIC util.cpp)
target_include_directories(util PUBLIC ../include)

This way, you will create a library util with its include path as "include". When processing target_link_libraries, CMake will make sure util's PUBLIC include path will be accessible by hello.

The benefit of setting util's (and not hello's) include path is that any target linking util will get the includes. Otherwise, each and every target using util would have to set the include path itself.