[CMake] Checking for empty string

[David Cole]

On Tue, Oct 25, 2011 at 6:31 AM, Rolf Eike Beer <eike at sf-mail.de> wrote:

> > hi there,
> >
> > how do I test if a string is empty ("") or not ?
> > IF( ${test} STREQUAL "" )
> > gives the wrong number of arguments if ${test} is ""
>
> if ("${test}" STREQUAL "")
> if (NOT test)
>
> What does not work (in 2.8.5) but should work according to the
> documentation:
>
> if (test STREQUAL "")
>

Ah, but this does work, and it works precisely according to the
documentation. Unfortunately, it is complicated, and confusing to human
beings, though...

The construct:

  if (test STREQUAL "")

is evaluated differently according to context. The context is whether or not
there is a variable defined that is named "test".

If there is a variable named test, then we do variable-value comparison:
  if (test STREQUAL "")
is equivalent to:
  if ("${test}" STREQUAL "")
(It is this way for backwards compatibility with IF commands that were
written before we even had ${test} variable expansion in the CMake language.
Years ago at this point...)

If there is no variable named test, then we do string literal comparison:
  if (test STREQUAL "")
is equivalent to:
  if ("test" STREQUAL "")

If you are intending string value comparison with variables, I always
recommend always using double quotes and always using the form:
  if ("${test}" STREQUAL "")

because it is unambiguous, and the meaning is clearer from reading it.


This is all illustrated by the following script:
(run with cmake -P to see the output does NOT go to the FATAL_ERROR
segments.....)

# test is not a defined variable at this point, so the following test is a
# simple string compare between the left and right arguments, which are
# "test" and ""
#
if (test STREQUAL "")
  message(FATAL_ERROR "unexpected: the string literal 'test' is the empty
string")
else()
  message("the string 'test' is not the empty string")
endif()


set(test "test contents")

# test is defined as a variable at this point, so the following test is a
# comparison of the string value of the variable test with the string
literal
# "", the empty string
#
if (test STREQUAL "")
  message(FATAL_ERROR "unexpected: the variable test is the empty string")
else()
  message("the variable test's value is NOT the empty string")
endif()


# Same variable-based if command now, but the value *is* now the empty
string:
#
set(test "")

if (test STREQUAL "")
  message("the variable test's value is the empty string")
else()
  message(FATAL_ERROR "unexpected: the variable test is NOT the empty
string")
endif()


HTH,
David



> Eike
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