Explain Use of ":" operator in c++ in the code snippet "int i:2;"

Question

Possible Duplicate:
What does this C++ code mean?

In the following C++ code

# include <stdio.h>
int main()
{
  struct clap
  {
   int i:2;
   int j:2;
   int k:3;
   }x={1,2,3};

  printf("%d %d %d",x.i,x.j,x.k);

  return 0;
}

On running the following code, I get the output 1 -2 3

Please Explain the meaning of the ":" operator in reference to the above code, and the reason for this strange output;

Possible duplicate: [/questions/14336994/c-structure-padding/14337041#14337062](http://stackoverflow.com/questions/14336994/c-structure-padding/14337041#14337062) — Theocharis K., Jan 15 '13 at 12:31

TemplateRex · Answer 1 · 2013-01-15T15:02:39.343

8

These indicate bit-fields with the length being denoted after the colon

  struct clap
  {
   int i:2; // length 2
   int j:2; // length 2
   int k:3; // length 3
   };

Bitfields save on space. Try computing sizeof(clap) and you'll find it is 4 bytes on gcc 4.7. The reason it is not 1 byte (2 + 2 + 3 = 7 bits < 1 byte), is that compilers also align structs on certain boundaries depending on the underyling type of the bitfields. E.g. changing int to short or char as the underlying type of the bitfields will reduce the total size of clap to respectively 2 and 1 byte(s) (on gcc 4.7 again).

This should be compared to storing 3 full integers typically takes 12 bytes (if an int is 4 bytes). OTOH, bit-fields can make your code slower because addressing the members entails shifting and unpacking the bitfields.

The sign problem arises because the 2-bits two's complement of 2 is equal to -2. Extending the code to int j:3 will output 2.

edited Jan 15 '13 at 15:02

answered Jan 15 '13 at 12:25

TemplateRex

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On running your code , the size of clap comes 4. Please explain how it is coming? – amol_beast Jan 15 '13 at 12:39
You can change the underlying type, using short instead of int, and look at the difference. – Marc Glisse Jan 15 '13 at 12:43
@amol_beast the size of clap is at least the size of the underlying bit-field type. E.g. having only `int i:1` as struct member, already gives `clap` size equal to 4 bytes. – TemplateRex Jan 15 '13 at 14:51

score 5 · Answer 2 · answered Jan 15 '13 at 12:25

5

The colon in the structure means that the members are bit fields. That is, each field only uses the specified number of bits.

That you get -2 for the field j is probably because printf treats is as a sign-extended integer.

answered Jan 15 '13 at 12:25

Some programmer dude

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Assuming your reason is true , then it should give -3 output for the value of k. Am I correct? – amol_beast Jan 15 '13 at 12:36
1

j has 2 bits, 10, the highest is 1, this is negative. k has 3 bits, 011, the highest is 0, this is non-negative. – Marc Glisse Jan 15 '13 at 12:44

Hui Zheng · Answer 3 · 2013-01-15T13:05:53.443

2

That is what's called bit field.

Update:

The reason that x.j was printed as -2: x.j is 2, but it only has 2 bits and the higher bit is 1, which will be taken as the sign bit by printf with %d format. To verify that, just change %d to %u and see what happens.

edited Jan 15 '13 at 13:05

answered Jan 15 '13 at 12:26

Hui Zheng

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what's wrong with my answer? – Hui Zheng Jan 15 '13 at 12:28
1

Not my down vote but I guess you need to describe more instead of short answers – billz Jan 15 '13 at 12:30
but I gave a link to wikipedia. – Hui Zheng Jan 15 '13 at 12:32
1

A link is good but the answer still doesn't contain much info in itself : still too short. You need to explain what is it, how to use it, use cases, side affects etc. then it becomes a good answer. – billz Jan 15 '13 at 12:35
+1 from me anyway. Enjoy your time on so. – billz Jan 15 '13 at 12:38
And, [links rot](http://en.wikipedia.org/wiki/Link_rot). – Sebastian Mach Jan 15 '13 at 12:39
@billz: ok. I believe sometimes is better to give a short hint to "the student" and he self read the documentation. But I guess HERE you are right... Good advice. – qPCR4vir Jan 15 '13 at 12:45
If it's so short HINT - use comment – billz Jan 15 '13 at 12:47
;-) thus is why I don’t put links, but the worlds to Google. Anyway, I don’t do anymore. – qPCR4vir Jan 15 '13 at 12:59

score 1 · Answer 4 · answered Jan 15 '13 at 12:49

Like other answers say it is bit field. One important note about them:

Thus C++0x treats all (non-zero-length) bit-fields in a contiguous sequence of bit-fields as part of a single memory location; an assignment to one conflicts with an assignment to any of the others. Such contiguous bit-fields should not be updated concurrently by different threads. Usually that means they should be protected by a single lock.

http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/threadsintro.html#bitfields

Explain Use of ":" operator in c++ in the code snippet "int i:2;"

4 Answers4