CMake compare to empty string with STREQUAL failed

Question

You ran into a rather annoying "it's not a bug, it's a feature" behavior of CMake. As explained in the documentation of the if command:

 The if command was written very early in CMake's history, predating the ${} 
 variable evaluation syntax, and for convenience evaluates variables named
 by its arguments as shown in the above signatures.

Well, the convenience turned out to be an inconvenience. In your example the the string "d" is treated as a variable named d by the if command. If the variable d happens to be defined to the empty string, the message statement will print "oops...", e.g.:

set (d "")
if("d" STREQUAL "")
  # this branch will be taken
  message("oops...")
else()
  message("fine")
endif()

This can give surprising results for statements like

if("${A}" STREQUAL "some string")

because there can be an unintended double expansion of the first argument if the variable A happens to be defined to a string which is also the name of a CMake variable, e.g.:

set (A "d")
set (d "some string")   
if("${A}" STREQUAL "some string")
  # this branch will be taken
  message("oops...")
else()
  message("fine")
endif()

Possible work-arounds:

You can add a suffix character to the string after the ${} expansion which prevents the if statement from doing the automatic evaluation:

set (A "d")
set (d "some string")
if("${A} " STREQUAL "some string ")
  message("oops...")
else()
  # this branch will be taken
  message("fine")
endif()

Do not use ${} expansion:

set (A "d")
set (d "some string")
if(A STREQUAL "some string")
  message("oops...")
else()
  # this branch will be taken
  message("fine")
endif()

To prevent unintended evaluation on the right side of STREQUAL use MATCHES with a CMake regular expression instead:

if(A MATCHES "^value$")
  ...
endif()

Addendum: CMake 3.1 no longer does double expansions for quoted arguments. See the new policy.

As of CMake 3.1, there are new rules variable expansions in if(). They are enabled if you either:

• set cmake_minimum_required(3.1) (or higher) at the top of you project file, or
• use a lower minimum version number but manually set policy CMP0054 to NEW.

Even in that case, it remains true is that the first argument to if is expanded with the value of a variable matching that name, if it exists:

set (d "")
if(d STREQUAL "")
  # this branch will be taken
  message("oops...")
else()
  message("fine")
endif()

However, this is now disabled if the first argument is quoted:

set (d "")
if("d" STREQUAL "")
  message("oops...")
else()
  # due to quotes around "d" in if statement,
  # this branch will be taken
  message("fine")
endif()

If you do want to test a variable's contents against a value, you can either use the classic unquoted syntax, or use the "${d}" syntax you suggested. Thanks to the new rules, this will never suffer the double-expansion problem mentioned in sakra's answer:

set (A "d")
set (d "some string")   
if("${A}" STREQUAL "d")
  # this branch will be taken
  message("fine")
elseif("${A}" STREQUAL "some string")
  message("oops...")
else()
  message("??")
endif()