CMake compare to empty string with STREQUAL failed

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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.

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