[SOLVED] Build a cpp file every time with cmake

Issue

I want a file that contains the build date to be build every time.

After some research I have tried many different approaches, but I don't understand them fully. I thought the following code should work, but it doesn't. It is written in a CMakeLists.txt file which is part of many cascaded CMakeLists.txt files.

[...]
add_library(${PROJECT_NAME} STATIC
        src/software_version.cpp [...]
)
[...]

set(VERSION_FILE "src/software_version.cpp")
add_custom_target(build_software_version ALL COMMAND ${CMAKE_COMMAND} -E touch ${VERSION_FILE})
#or
add_custom_command(TARGET ${PROJECT_NAME} PRE_BUILD  COMMAND ${CMAKE_COMMAND} -E touch ${VERSION_FILE})

Edit:
I'm using the predefined __ DATE__ in a method in that file and I've found out that it wont be up-to-date if I don't build the file every time. If I just make a small change to the file so the file is beeing build again, the date is correct. I was hoping to achive that with the touch command. So I was expecting to see the note that the file was build in the build-log. (An other file shows up there every time too)
At this point the first command compiles, but doesn't change anything. The second one throws an error at building time. [error]: https://i.stack.imgur.com/W5Cle.png

Solution

Your solution with add_custom_target almost worked, there are just two things which need to be added:

1. the custom target is run from the build directory... where no source file with that name is to be found. Prefixing CMAKE_CURRENT_SOURCE_DIR fixes that. It would also be possible to set the WORKING_DIRECTORY parameter of add_custom_target.
2. a dependency needs to be added as pointed out by Jarod42 in the comments.

I believe the following code does what you want:

add_custom_target(build_software_version 
    COMMAND ${CMAKE_COMMAND} -E touch src/software_version.cpp
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
    )
add_library(${PROJECT_NAME} STATIC src/software_version.cpp)
add_dependencies(${PROJECT_NAME} build_software_version)

---

Some more thoughts

While the text above answers the question, it's not necessarily the best solution for the underlying problem. To have the current date compiled into the binary, I'd write the date into a file using CMake rather than relying on the __DATE__ preprocessor variable. If done right, this would force rebuilds only once per day.

Answered By - Friedrich

Answer Checked By - Katrina (WPSolving Volunteer)