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

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 through the GCC preprocessor manual and specification guide and can't find anything about it.

(1) #ifndef BOOST_CONFIG_HPP
(2) #  include <boost/config.hpp>
(3) #endif
(4) #
(5) #if defined(BOOST_HAS_PRAGMA_ONCE)
(6) #  pragma once
(7) #endif

On line 4, there is nothing after the pound sign. What effect does this have? Is it defined in the C preprocessor (CPP) specification?

As Boost is a cross-platform library, I would assume that any CPP should parse it correctly. What would the effect/side-effects be of having random pound/hash signs throughout the code?

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.

It makes the source code look pretty, that's all.

Highlights the fact that the whole block is a preprocessor section.

And indeed, both the C and C++ preprocessors must ignore # on a line.

Always check an authoritative source instead of relying on other resources. C is standardised as ISO 9899::2011, C++ also has an ISO standard. Both are well accepted and the final drafts available by a short search. The C standard states in 6.10.7 (C++ has much the same text):

> A preprocessing directive of the form > > # new-line > > has no effect.

This is a null directive, as much as an ; without a preceeding expression in the core-language is a null statement .

For the preprocessor it is just for formatting/readability to highlight that the lines belong semantically together. (the semicolon OTOH is semantically relevant).