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 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?

Just curious: is CPP a valid abreviation for C preprocessor? It quite confused me when reading OP. — dhein, Feb 05 '16 at 11:03
@Zaibis Eg. the executable name of the preprocessor in the GCC suite is "cpp" (while the compilers are gcc and g++) — deviantfan, Feb 05 '16 at 11:14
Not entirely. See the first sentence of https://en.wikipedia.org/wiki/C_preprocessor — P45 Imminent, Feb 05 '16 at 15:26
@djeidot "cpp" is ambiguous. That is why people use "c++", or "cxx" (x looks like + turned 45 degrees) when referring to C-Plus-Plus. — Mike Ounsworth, Feb 05 '16 at 16:30
@djeidot No, CPP is the C-PreProcessor. It existed way before C++ even existed. — Leandros, Feb 06 '16 at 23:39
@djeidot, [with `Makefile`s](http://stackoverflow.com/a/2756681/912144) and autotools, `CPPFLAGS` refers to the preprocessor. — Shahbaz, Feb 07 '16 at 07:27

score 189 · Accepted Answer · edited Jun 20 '20 at 09:12

189

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.

edited Jun 20 '20 at 09:12

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answered Feb 04 '16 at 17:27

Jonathan Wakely

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This doesn't explain the purpose of using it, though, nor give the rationale for its existence. – StellarVortex Feb 09 '16 at 19:23
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_"What effect does this have? Is it defined in the C preprocessor (CPP) specification? ... What would the effect/side-effects be of having random pound/hash signs throughout the code?"_ That's what I answered. It has no effect, but I didn't want to speculate on the author's reason for using it. I have done so now. – Jonathan Wakely Feb 09 '16 at 19:47
You have been given correct answers that it means nothing to the preprocessor; I'm going to speculate that it may help some other program (such as an IDE or LINT) keep a block of directives together as a logical unit. Some IDEs let programmers expand or collapse blocks of text to help them keep track of the logical structure of the file. – Spencer Aug 12 '17 at 01:50

score 110 · Answer 2 · answered Feb 04 '16 at 17:22

110

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.

answered Feb 04 '16 at 17:22

P45 Imminent

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And also makes navigating easier in some text editors (e.g., `{` or `}` in vim). – wchargin Feb 04 '16 at 19:43
@WChargin, that depends on how you look at it. If you want to navigate to between the two preprocessor blocks, adding `#` would prevent you from using `{` or `}`. In fact, it may be easier to press `}` twice to jump over the block (in the OP's example) than not be able to jump to the middle of the two blocks. – Shahbaz Feb 07 '16 at 07:29
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@Shahbaz Certainly! My rule of thumb is "keep logical units together," so that "paragraph" really means "idea." I would follow this rule with preprocessor declarations, too. Of course, it is a matter of personal style to a large degree. – wchargin Feb 07 '16 at 14:08

score 47 · Answer 3 · edited Jun 20 '20 at 09:12

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).

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

3 Answers3

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