CMake using add_custom_target and add_dependencies

CMakeLists.txt

cmake_minimum_required(VERSION 3.20)

project(deps)

file(WRITE ${CMAKE_BINARY_DIR}/library.cpp "
#include <fstream>
#include <string>

std::string message()
{
    std::ifstream is(\"message.txt\");
    return std::string(std::istreambuf_iterator<char>(is), std::istreambuf_iterator<char>());
}
")

file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
file(WRITE ${CMAKE_BINARY_DIR}/lib/message.txt "Hello Library World!")

file(WRITE ${CMAKE_BINARY_DIR}/main.cpp "
#include <iostream>
#include <string>

std::string message();

int main()
{
    std::cout << message() << std::endl;

    return 0;
}
")

add_library(msg ${CMAKE_BINARY_DIR}/library.cpp)

add_custom_target(msg_dep COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_BINARY_DIR}/lib/message.txt ${CMAKE_BINARY_DIR}/message.txt)
add_dependencies(msg msg_dep)

add_executable(exe ${CMAKE_BINARY_DIR}/main.cpp)
target_link_libraries(exe PRIVATE msg)

add_custom_target(run COMMAND exe)