In cmake how to create a regular file using the target generated - cmake

Question

I want to create a regular file using the target generated, here is the sample code:
cmake_minimum_required(VERSION 3.2 FATAL_ERROR)
add_executable (write_fields #write_fields will create test.dat
main.cpp
)
add_custom_command (
OUTPUT test.dat
DEPENDS write_fields
COMMAND ${CMAKE_BINARY_DIR}/write_fields
VERBATIM
)
But it seems that the custom command never get executed
UPDATE:
The following code doesn’t work either
cmake_minimum_required(VERSION 3.2 FATAL_ERROR)
project(myproj)
add_executable (write_fields
main.cpp
)
add_custom_command (
OUTPUT ${CMAKE_SOURCE_DIR}/test.dat
DEPENDS write_fields
COMMAND ${CMAKE_BINARY_DIR}/write_fields
VERBATIM
)
add_custom_target(myproj DEPENDS ${CMAKE_SOURCE_DIR}/test.dat)

Answer 1

Please read documentation carefully:
If COMMAND specifies an executable target (created by the add_executable() command) it will automatically be replaced by the location of the executable created at build time.
in other words you can use just a plain target name to execute it:
cmake_minimum_required(VERSION 3.2 FATAL_ERROR)
add_executable (
write_fields #write_fields will create test.dat
main.cpp
)
add_custom_command(
COMMAND write_fields
OUTPUT test.dat
DEPENDS write_fields
VERBATIM
)

Answer 2

The second signature adds a custom command to a target such as a
library or executable. This is useful for performing an operation
before or after building the target. The command becomes part of the
target and will only execute when the target itself is built. If the
target is already built, the command will not execute.
add_custom_command(TARGET target
PRE_BUILD | PRE_LINK | POST_BUILD
COMMAND command1 [ARGS] [args1...]
[COMMAND command2 [ARGS] [args2...] ...]
[WORKING_DIRECTORY dir]
[COMMENT comment] [VERBATIM])
This defines a new command that will be associated with building the
specified target. When the command will happen is determined by which
of the following is specified:
PRE_BUILD - run before all other dependencies
PRE_LINK - run after other dependencies
POST_BUILD - run after the target has been built
cmake_minimum_required(VERSION 3.2 FATAL_ERROR)
add_executable (write_fields
main.cpp
)
add_custom_command (
TARGET write_fields
POST_BUILD
COMMAND write_fields
VERBATIM
)