Checking for CMake Variables

Question

I have the following CMake code snippet in my CMakeLists.txt:

cmake_minimum_required(VERSION 2.8.8 FATAL_ERROR)  message(STATUS "Before setting - ${MY_VARIABLE}")  # first check if(NOT DEFINED ${MY_VARIABLE})   set(MY_VARIABLE true) endif(NOT DEFINED ${MY_VARIABLE})  message(STATUS "After setting - ${MY_VARIABLE}")  # second check if(NOT DEFINED ${MY_VARIABLE})   message(STATUS "What - ${MY_VARIABLE}") endif(NOT DEFINED ${MY_VARIABLE}) 

The output from CMake configuration is:

-- The C compiler identification is GNU 4.8.4 -- The CXX compiler identification is GNU 4.8.4 -- Check for working C compiler: /usr/local/bin/cc -- Check for working C compiler: /usr/local/bin/cc -- works -- Detecting C compiler ABI info -- Detecting C compiler ABI info - done -- Check for working CXX compiler: /usr/local/bin/c++ -- Check for working CXX compiler: /usr/local/bin/c++ -- works -- Detecting CXX compiler ABI info -- Detecting CXX compiler ABI info - done -- Before setting -  -- After setting - true -- What - true -- Configuring done -- Generating done -- Build files have been written to: /tmp/build 

Question: Why does the second check for variable definition report the variable is not defined even though it is defined? Surprisingly, the value of the variable is also printed correctly!

Answer 1

There is a difference between: if(NOT DEFINED VAR_NAME) and if(NOT DEFINED ${VAR_NAME})

The first one refers to the variable and the other to its content.