why size of a struct with only a bit is 4 byte but not 1 byte

Question

I think it should be 1. Because 1 byte is enough to hold this BOOL. And if I comment "BOOL b:1", it becomes a empty struct, and its size is 1 now.

#include <iostream>
#include <string>
using namespace std;

enum BOOL { FALSE=0, TRUE=1 };
struct A {
  BOOL b:1;
};

int main()
{
  std::cout << sizeof(A)  << std::endl; //output 4.
}

http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member — sashang, Sep 15 '15 at 02:18
There's a very good chance the enum was 4 bytes long, and the bitfield was also 4 bytes long. For example, I tested it in VS2013 and enum defaults to int, and bitfields reserve the same amount of space as the type specified. — Weak to Enuma Elish, Sep 15 '15 at 02:36
`unsigned long long ull : 1` is 8 bytes, whereas `char c : 1` is 1 byte. — Weak to Enuma Elish, Sep 15 '15 at 02:37

score 2 · Answer 1 · answered Sep 15 '15 at 02:16

2

Padding.

If you want the struct to be "packed" (good for memory use, TERRIBLE for performance), you can ask for that in a compiler-specific nonstandard way.

answered Sep 15 '15 at 02:16

Borealid

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I know there is padding. But one byte is enough to do the padding I think. And do you mean that a empty struct will have terrible performance. I don't think so. – Jack Chin Sep 15 '15 at 02:22
Read up on "access alignment". Most CPUs cannot access an arbitrary two bytes in a performant fashion. – Borealid Sep 15 '15 at 05:32

why size of a struct with only a bit is 4 byte but not 1 byte

1 Answers1