What is the purpose of a single pound/hash sign (#) on its own line in the C/C++ preprocessor?

Question

I have been looking at the Boost libraries source code, and I have noticed that often there are single pound signs without any preprocessor directives attached to them. I read t

Answer 1

A # on its own on a line has no effect at all. I assume it's being used for aesthetic value.

The C standard says:

6.10.7 Null directive

Semantics

A preprocessing directive of the form

# new-line

has no effect.

The C++ standard says the same thing:

16.7 Null directive [cpp.null]

A preprocessing directive of the form

# new-line

has no effect.