CMake undefined variable compare

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")

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.