Introduction
Bit-flags are used in C and C++ to save memory by allowing you to pack multiple boolean variables into a single integer variable.
While they are very good at this, the syntax for using them isn’t exactly the easiest to read, so I thought I’d share a trick we use at work to deal with bit-flags.
The Code
struct Flags8
{
union
{
uint8_t allFlags;
struct
{
bool flag1 : 1;
bool flag2 : 1;
bool flag3 : 1;
bool flag4 : 1;
bool flag5 : 1;
bool flag6 : 1;
bool flag7 : 1;
bool flag8 : 1;
};
};
};
The code above shows a struct containing a union between an 8-bit unsigned integer, and another struct containing a series of bit-wide booleans (that’s what the : 1 does).
The union means that the allFlags variable contains the combined bit-flag value of all the booleans in the inner struct. This means that you can only have as many flags in your struct as you can fit bits in your integer; for an 8-bit integer, this is 8; for a 16-bit integer it would be 16; for a 32-bit integer, it would be 32.
You would use the above struct like so.
Flags8 flags8;
flags8.allFlags = 0;
flags8.flag2 = true;
assert(sizeof(Flags8) == sizeof(uint8_t));
assert(!flags8.flag1);
assert(flags8.flag2);