How the code of struct works,Please explain the code below

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C++

struct abc
{
  int a:3;
  int b:3;
  int c:2;
}
void main()
{
   struct abc a=(2,3,4)
   printf(" %d %d %d",a.a,a.b,a.c);
   gethc();
}

Posted 29-Feb-12 23:42pm

Balmukund Dewangan

Updated 1-Mar-12 0:12am

Richard MacCutchan

v2

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Chandrakantt · Answer 1 · 2012-03-01T00:06:00

Modified code which runs

C#

struct abc
{
    int a:3;
    int b:3;
    int c:2;
};

int _tmain(int argc, _TCHAR* argv[])
{
    struct abc a = {2,3,4};
    printf(" %d %d %d, %d",a.a,a.b,a.c,sizeof(struct abc));
    return 0;
}

So here a is assigned 3 bits, b is assigned 3 bits and c is assigned 2 bits in your structure. Now when you give value to a structure variable.
Values provided in bit

a = 10 (2 in binary) (take 2 bit space out of three)
b = 11 (3 in binary) (take 2 bit space out of three)
c = 100 (4 in binary) (needs 3 bit space but have 2 bit only)

so value of a and b will be printed correctly, but value of c will be 0, as third bit from (100) that is 1 is not stored.

OriginalGriff · Answer 2 · 2012-03-01T00:10:00

This is a special kind of struct, called a bitfield.
The whole structure is stored in a single word, with the number after the colon indicating how many bits of that word are taken by each element. So "a" is 3 bits (0-7 inclusive), "b" is 3 bits, and "c" is 2 bits (0-3 inclusive). The added complication is that int is a signed type, so your value range is a lot smaller...
Your code won't compile, so there is no point in explaining it in detail! Fix your syntax errors first, then ask about specific problems.

CPallini · Answer 3 · 2012-03-01T00:11:00

C++

struct abc
{
int a:3;
int b:3;
int c:2;
}

Defines (note: the missing semicolon is a syntax error) three bit fields in the struct abc. All share the same type, namely signed integer, a and b are three bits long (they may hold the values {-4,-3,-2,-1,0,1,2,3}) while c is two bits long (it may hold the values {-2,-1,0,1}).

C++

struct abc a=(2,3,4)

The above line (again, the missing semicolon is a syntax error) initializes the instance a of the struct abc this way:

C++

a.a = 2;
a.b = 3;
a.c = 4; // NOTE: 4 is an out of range value for c

C++

printf(" %d %d %d",a.a,a.b,a.c);

The above line prints the values of the bit fields.

C++

gethc();

I don't know what it is. Do you mean getch(); instead?

How the code of struct works,Please explain the code below

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