In CMake, undefined variables can evaluate as false in simple if(x) statements. For comparison operations like LESS GREATER, undefined variables do not operate like “false”.

if(x LESS 1)
  message(FATAL_ERROR "undefined not less 1")
endif()

No matter what value is compared to undefined variable “x”, the if() statement will not be true. As in most programming languages, a key best practice for CMake is to ensure variables are defined with a proper default value.