what does "##" symbol mean in C++

Question

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Macro expansion in C++
What is the purpose of the ## operator in C++, and what is it called?

What does "##" symbol mean in C++? I came across with it while I was reading someone's source code. More specifically, what does this statement mean:

if ( v > ## = 0.1 * threshold )

@Tomalak: :) Google is not friendly to ##.. hard to search for this one — , Jul 11 '11 at 01:11

score 7 · Accepted Answer · answered Jul 11 '11 at 01:01

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In a #define macro, ## is a preprocessor token, which says to paste the surrounding two things together. So assuming you saw this within a #define, it is a very strange way of writing

if ( v >= 0.1 * threshold )

If you already subsituted either the > or =, it's not quite so strange. Just a somewhat strange trick.

answered Jul 11 '11 at 01:01

aschepler

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Only if the code was part of a macro definition. There was no evidence of that in the question. – Lightness Races in Orbit Jul 11 '11 at 01:01
(And [it _does_ have to be part of a macro definition](http://www.ideone.com/BeAbn).) – Lightness Races in Orbit Jul 11 '11 at 01:03
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@Tomalak: If the code compiles, it must be part of a macro definition. I think it's reasonable to guess that the code compiles. – Dietrich Epp Jul 11 '11 at 01:04
It actually is a macro definition, thanks guys. – small_potato Jul 11 '11 at 01:05
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@small_potato: You _must_ include that sort of thing in your post. It's utterly crucial. What you called a "statement" is in fact nothing of the sort in your code. – Lightness Races in Orbit Jul 11 '11 at 01:05
@Tomalak: it's most useful a part of a macro, but I don't believe it's restricted to use in the body of a macro. Do you have a standards reference? (If only because I'm not finding any free ones on a quick earch.) – geekosaur Jul 11 '11 at 01:09
@geekosaur: I don't need one. I demonstrated that it's invalid outside of a macro definition. And, if you're still not convinced, `##` is defined in `cpp.concat` and not given semantics elsewhere. – Lightness Races in Orbit Jul 11 '11 at 01:11
@geekosaur: Feel free to decode the C++ grammar if you _still_ need proof. You'll find the most convincing argument there. – Lightness Races in Orbit Jul 11 '11 at 01:14

score 2 · Answer 2 · answered Jul 11 '11 at 01:05

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It's a preprocessor token:

http://msdn.microsoft.com/en-us/library/09dwwt6y%28v=vs.80%29.aspx

answered Jul 11 '11 at 01:05

Joe

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score 1 · Answer 3 · answered Jul 11 '11 at 01:00

It means nothing. This is an error.

Are you sure that the code after ## isn't a comment, and that the conditional is not continued properly on the next ensuing line?

Alternatively, the code you pasted may be part of a line that constitutes a macro definition (but you showed no evidence of that, so I won't answer that different question here).

_{(You see how context matters in questions like this?)}

score 1 · Answer 4 · answered Jul 11 '11 at 01:02

## tells cpp to paste the tokens before and after it together. In this case, I would have to guess that for some reason the programmer needed to avoid having an actual >= token, possibly to avoid confusing some other preprocessor (documentation generator or whatever), so has cpp assemble the >= from its components.

what does "##" symbol mean in C++

4 Answers4