Is using '!!' in C/C++ good practice and is it common?

Question

Possible Duplicate:
Double Negation in C++ code

As far as I know, no C/C++ books tutorials or manuals mention this technique. Maybe because it's just a tiny little thing, not worth mentioning.

I use it because C/C++ mixes bool type with int, long, pointer, double etc...together. It's very common to need to convert a non-bool to bool. It's not safe to use (bool)value to do that, so I use !! to do it.

Example:

bool bValue = !!otherValue;

This matter was already discussed here: http://stackoverflow.com/questions/248693/double-negation-in-c-code and http://stackoverflow.com/questions/8617090/double-negation-in-c-is-it-guaranteed-to-return-0-1 — Juliano, May 02 '12 at 00:14
@MarkByers: Yet there are concepts that apply equally well to both. This is not one of them however... — Ed S., May 02 '12 at 00:28
It's pretty ugly. And generally not needed, at least not in C++. There is an inbuilt conversion for integer to bool, T* to bool, etc. — dave, May 02 '12 at 00:43
If you don't want to use a C-style cast like `(bool)value`, then try a C++-style conversion, like `bool(value)` :) — Karl Knechtel, May 02 '12 at 01:12
@KarlKnechtel: That is still a c-style cast. C++ cast would be `static_cast(value)` — Inverse, May 03 '12 at 18:34

Ed S. · Answer 1 · 2012-05-02T00:27:45.257

19

It's fine, any C or C++ programmer should recognize it, but I would prefer something more explicit like

(x != 0)

edited May 02 '12 at 00:27

answered May 02 '12 at 00:11

Ed S.

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+1 Clearer code is better code – Juliano May 02 '12 at 00:16

score 10 · Accepted Answer · answered May 02 '12 at 00:15

10

I think !! is quite clear in comparison to some of the other choices such as :

if (foo)
  bar = 1;
else
  bar = 0;

or bar = foo ? 1 : 0;

Since !! does exactly one thing, I find it very unambiguous.

answered May 02 '12 at 00:15

sarnold

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Well, I agree that it's ok, but `!!foo` is *less* ambiguous than `foo ? 1 : 0`? I can't agree with that. – Ed S. May 02 '12 at 00:16
@EdS. Heh, in fact, I posted mainly to counter your post -- I disagreed with your point of view, and figured we'd let the votes decide. So far you're well in the lead. :) – sarnold May 02 '12 at 00:17
Well, you came into the game late, so give it some time :D – Ed S. May 02 '12 at 00:18
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I *never* use this in C++ (I cast or implicitly convert to `bool`), but I use it fairly often in C. To my eye, `!!x` is so obvious that I see a `!!` operator, not just two `!`s. – Robᵩ May 02 '12 at 00:20
@Robᵩ: Some people do actually think that `!!` is its own operator and get confused. =/ – jamesdlin May 02 '12 at 01:16
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Some people also think `-->` is its own operator... ;-) – R.. GitHub STOP HELPING ICE May 02 '12 at 01:38
@jamesdlin: not people with any amount of experience with the language. – Ed S. May 02 '12 at 04:33

score 5 · Answer 3 · answered May 02 '12 at 00:11

5

In this exact case:

bool bValue = !!otherValue;

you don't need to write !!. It will work fine without them:

bool bValue = otherValue;

I think in most cases implicit casting will be nice.

answered May 02 '12 at 00:11

Pavel Strakhov

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Implicit casting (when it can cause data truncation) causes compiler warnings, explicit casting (via operators in this case) does not. – ildjarn May 02 '12 at 00:44

Is using '!!' in C/C++ good practice and is it common?

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