Cmake add_custom_command misunderstanding - shell

Question

I'm learning CMake and struggling with learning the: add_custom_command function.
I'm sorry if this is a basic question but the online documentation didn't help me much.
For this snippet:
add_executable (creator creator.cpp)
get_target_property (creator EXE_LOC LOCATION) # get creator.cpp location
add_custom_command(
OUTPUT ./created.cpp # creates 'created.cpp' at the specified path
DEPENDS creator # specifies files on which the command depends
COMMAND ${EXE_LOC} # executes this command
ARGS ./created.cpp
)
add_executable(FOO ./created.c)
I can intuitively realise what's going on there, however, I do not understand why each instruction from the body of add_custom_command is needed. Here is how I understand it (please correct me where I'm wrong):
Executable creator is created in the current working dir using creator.cpp
EXE_LOC variable is used to store the path of the created executable
add_custom_command:
OUTPUT specifies that a created.cpp file will be created in the current
working directory.
DEPENDS: specifies that this newly created .cpp file depends on the
previously created executable. But why do we need to specify this? Is it
mandatory to do so and if not what happens if I don't specify this
COMMAND: ${EXE_LOC}: This I don't understand. I assume when the script
reaches this point some sort of command will be executed. But what exactly is going to get executed here? ./creator maybe? The documentation specifies that:
If COMMAND specifies an executable target (created by ADD_EXECUTABLE) it will automatically be replaced by the location of the executable created at build time.
But I don't really understand this.
ARGS: I don't understand what this is supposed to do and why do we need it.
It really confuses me that we pass the newly created file as an argument to a command whose purpose is to create that particular file.
Please clear this for me if possible.
Thank you for reading my long post.

Answer 1

It is convinient to think about COMMAND option as a command line:
First, you need to type the path to the program to be executed. In given case it is a path to creator executable. Then, you type arguments for given command. In given case, the only argument is a path to the created.cpp file. This is what will be executed:
<path-to-creator-executable> <path-to-cpp-file>
Option
DEPENDS creator
in given case is perfectly described in CMake documentation for add_custom_command:
If DEPENDS specifies any target (created by the add_custom_target(), add_executable(), or add_library() command) a target-level dependency is created to make sure the target is built before any target using this custom command. Additionally, if the target is an executable or library a file-level dependency is created to cause the custom command to re-run whenever the target is recompiled.
In short, this means that before creator executable will be run as specified in the COMMAND option, the executable will be created and updated if needed.