Questions

• “:” (colon) in C/C++ struct, what does it mean?
• What does C++ struct syntax “a : b” mean?
• What does :1 and :8 mean?

Example #1

https://en.wikipedia.org/wiki/Bit_field

struct BoxProps
{
  unsigned int  opaque       : 1;
  unsigned int  fill_color   : 3;
  unsigned int               : 4; // fill to 8 bits
  unsigned int  show_border  : 1;
  unsigned int  border_color : 3;
  unsigned int  border_style : 2;
  unsigned char              : 0; // fill to nearest byte (16 bits)
  unsigned char width        : 4, // Split a byte into 2 fields of 4 bits
                height       : 4;
};

Example #2

https://en.wikipedia.org/wiki/C_syntax#Bit_fields

struct f {
    unsigned int  flag : 1;  /* a bit flag: can either be on (1) or off (0) */
    signed int    num  : 4;  /* a signed 4-bit field; range -7...7 or -8...7 */
    signed int         : 3;  /* 3 bits of padding to round out to 8 bits */
} g;

Example #3

https://en.cppreference.com/w/cpp/language/bit_field

struct S
{
    // will usually occupy 2 bytes:
    // 3 bits: value of b1
    // 2 bits: unused
    // 6 bits: value of b2
    // 2 bits: value of b3
    // 3 bits: unused
    unsigned char b1 : 3, : 2, b2 : 6, b3 : 2;
};

Example #4

https://learn.microsoft.com/en-us/cpp/cpp/cpp-bit-fields?view=msvc-170

// bit_fields1.cpp
// compile with: /LD
struct Date {
   unsigned short nWeekDay  : 3;    // 0..7   (3 bits)
   unsigned short nMonthDay : 6;    // 0..31  (6 bits)
   unsigned short nMonth    : 5;    // 0..12  (5 bits)
   unsigned short nYear     : 8;    // 0..100 (8 bits)
};

Example #5: union bit fields & uint8

https://stackoverflow.com/questions/36255992/union-bit-fields

union {
uint8_t raw;
struct {
    uint8_t a : 1;
    uint8_t b : 2;
    uint8_t padding : 5;
};
} U;