c++ - How does compiler determine the size of a bitfield struct? -

for example:

struct {     uint32_t    forecolor_ : 32;     uint32_t    backcolor_ : 32;     uint16_t    lfheight_ : 16;     uint16_t    flags_: 4;     bool    lfbold_: 1;     bool    lfitalic_: 1;     bool    lfunderline_: 1;     bool    lfdashline_: 1;      bool    lfstrike_: 1;     bool    lfsubscript_: 1;     bool    lfsuperscript_: 1; }; 

is 16 bytes but

struct {     uint32_t    forecolor_ : 32;     uint32_t    backcolor_ : 32;     uint16_t    lfheight_ : 16;     uint8_t     flags_: 4;     bool    lfbold_: 1;     bool    lfitalic_: 1;     bool    lfunderline_: 1;     bool    lfdashline_: 1; //for ime      bool    lfstrike_: 1;     bool    lfsubscript_: 1;     bool    lfsuperscript_: 1; }; 

is 12 bytes long.

i thought flags_ should have same length, seems not.

why?

the standard (9.6 of the working draft) says this:

specifies bit-field; length set off bit-field name colon. bit-field attribute not part of type of class member. constant-expression shall integral constant-expression value greater or equal zero. constant-expression may larger number of bits in object representation ( 3.9 ) of bit-field’s type; in such cases bits used padding bits , not participate in value representation ( 3.9 ) of bit-field. allocation of bit-fields within class object implementation-defined. alignment of bit-fields implementation-defined. bit-fields packed addressable allocation unit. [ note: bit-fields straddle allocation units on machines , not on others. bit-fields assigned right-to-left on machines, left-to-right on others. —end note]

(my emphasis)

so depend on compiler. appears happening in case - , describe normal behaviour - is combining bitfields of same type , packing structure 4 byte boundary, in first case have:

struct {     uint32_t    forecolor_ : 32; // 4 bytes (total)     uint32_t    backcolor_ : 32; // 8 bytes     uint16_t    lfheight_ : 16;  // 10 bytes     uint16_t    flags_: 4;       // 12 bytes     bool    lfbold_: 1;          // 13 bytes     bool    lfitalic_: 1;     bool    lfunderline_: 1;     bool    lfdashline_: 1;      bool    lfstrike_: 1;     bool    lfsubscript_: 1;     bool    lfsuperscript_: 1;   // still 13 bytes }; 

which padded 16 bytes, , in second have:

struct {     uint32_t    forecolor_ : 32;  // 4 bytes (total)     uint32_t    backcolor_ : 32;  // 8 bytes     uint16_t    lfheight_ : 16;   // 10 bytes     uint8_t     flags_: 4;        // 11 bytes     bool    lfbold_: 1;           // 12 bytes     bool    lfitalic_: 1;     bool    lfunderline_: 1;     bool    lfdashline_: 1;      bool    lfstrike_: 1;     bool    lfsubscript_: 1;     bool    lfsuperscript_: 1;    // still 12 bytes }; 

which needs no padding , stays @ 12 bytes.