If a heap variable is destroyed, any remaining pointer variable or object reference that still refers to it is said to contain a dangling reference. Unlike lower level languages such as C, dereferencing a dangling reference will not crash or corrupt your IDL session. It will, however, fail with an error message. For example:
;Create a new heap variable. A = PTR_NEW(23) ;Print A and the value of the heap variable A points to. PRINT, A, *A
;Destroy the heap variable. PTR_FREE, A ;Try to print again. PRINT, A, *A
% Invalid pointer: A. % Execution halted at: $MAIN$
There are several possible approaches to avoiding such errors. The best option is to structure your code such that dangling references do not occur. You can, however, verify the validity of pointers or object references before using them (via the PTR_VALID or OBJ_VALID functions) or use the CATCH mechanism to recover from the effect of such a dereference.