How slow are bit fields in C++
- by Shane MacLaughlin
I have a C++ application that includes a number of structures with manually controlled bit fields, something like
#define FLAG1 0x0001
#define FLAG2 0x0002
#define FLAG3 0x0004
class MyClass
{
'
'
unsigned Flags;
int IsFlag1Set() { return Flags & FLAG1; }
void SetFlag1Set() { Flags |= FLAG1; }
void ResetFlag1() { Flags &= 0xffffffff ^ FLAG1; }
'
'
};
For obvious reasons I'd like to change this to use bit fields, something like
class MyClass
{
'
'
struct Flags
{
unsigned Flag1:1;
unsigned Flag2:1;
unsigned Flag3:1;
};
'
'
};
The one concern I have with making this switch is that I've come across a number of references on this site stating how slow bit fields are in C++. My assumption is that they are still faster than the manual code shown above, but is there any hard reference material covering the speed implications of using bit fields on various platforms, specifically 32bit and 64bit windows. The application deals with huge amounts of data in memory and must be both fast and memory efficient, which could well be why it was written this way in the first place.
