How to add empty element to CMake list? - cmake

Question

There is CMake script:
unset(FOO)
list(APPEND FOO "")
list(APPEND FOO "")
list(APPEND FOO "")
list(APPEND FOO "")
list(LENGTH FOO SIZE)
message(FATAL_ERROR ${SIZE})
Expectation - 4. Reality - 0
I've found a workaround, but it looks like a kludge:
set(FOO "dummy")
set(FOO "${FOO};")
set(FOO "${FOO};")
set(FOO "${FOO};")
set(FOO "${FOO};")
list(REMOVE_AT FOO 0)
message(FATAL_ERROR ${SIZE})
The example a little bit simpler than my task, so, please, don't suggest writing set(FOO ";;;"):) I get elements from outside, and some of them may be empty. Something like this:
function(do_smth LIST_WITH_EMPTY_ITEMS)
unset(RESULT)
foreach(X IN LISTS LIST_WITH_EMPTY_ITEMS)
if(${CONDITION})
list(APPEND RESULT "${X}")
endif()
endforeach()
endfunction()

Answer 1

It doesn't seem to work with empty lists, but if you initialize the list with some dummy value, as you did in your second example set(FOO "dummy"), you can continue to use FOO as a list and also append empty elements to it. It's basically your desired approach and workaround combined:
set(FOO "dummy")
list(APPEND FOO "")
list(APPEND FOO "")
list(APPEND FOO "")
list(APPEND FOO "")
list(POP_FRONT FOO)
list(LENGTH FOO SIZE)
message("content: (${FOO})")
message("sizt: ${SIZE}")
The output of this is:
content: (;;;)
size: 4