Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
This makes it more clear that a null check is intended. Avoiding the
use of a pointer as a test condition alse increases the type-safety
of the comparisons.
(This is also MISRA C 2012 rules 14.4 and 11.9)
