Variables set with PARENT_SCOPE are empty in the corresponding child scope. Why?

Question

Consider the following minimal example:

.
├── bar
│   └── CMakeLists.txt
└── CMakeLists.txt

where ./CMakeLists.txt is

Answer 1

Each variable in the cmake has it's own scope so it is dangerous use case where a variable automatically propagates in a child context, because it can interfere with it from a parent scope!

But you can set just another variable in a child scope to test it later instead of rereading a parent one:

./bar/CMakeLists.txt:

set( BAR "Exists in parent scope only." PARENT_SCOPE )
set( _somerandomid_BAR "Exists in child scope only.")
message( STATUS "Variable BAR in ./bar/ = ${_somerandomid_BAR}" )

Now, if you have loops in your code, then you can test both variables:

foreach(...)
  ...
  # read a variable token name and token value, for example, from a configuration file
  set(my_var_name_token ...)
  set(my_var_value_token ...)
  ...
  # parse a variable name and value tokens into a real name and value
  set(my_var_name ...)
  set(my_var_value ...)
  ...
  if (DEFINED ${my_var_name})
    if (DEFINED local_${my_var_name})
      message("has been defined before and was resetted");
    else()
      message("has been defined before and was not resetted");
    endif()
  else()
    if (DEFINED local_${my_var_name})
      message("has not been defined before and was setted");
    else()
      message("has not been defined before and was not touched");
    endif()
  endif()
  ...
  # sets parsed name and value into cmake context
  set(${my_var_name} "..." PARENT_SCOPE)

  # Do save all values has been setting from this function to differently compare and 
  # validate them versus already existed before:
  # 1. If has been set before, then must be set to the same value, otherwise - error
  # 2. If has not been set before, then should set only once, otherwise - ignore new 
  # value (a constant variable behaviour)
  set(local_${my_var_name} "...")
  ...
endforeach()