c - sizeof anonymous nested struct -

suppose have structure i'm using model various packet formats:

#define maxpacket 20  typedef struct {     u8 packetlength;     union {         u8 bytes[maxpacket];         struct {             u16 field1;             u16 field2;             u16 field3;         } format1;         struct {             double value1;             double value2;         } format2;     }; } packet; 

i can expect sizeof(packet) 21. there way like:

sizeof(packet.format2) 

? i've tried that, compiler not happy. obviously, pull format1 out separate typedef , sizeof(format1). i'm curious if have through of that. hierarchical composition of formats. gcc on 8bit processor.

i'm equally interested if there's way use nested type. if have lot of

apacketpointer->format2.value1; // not onerous, if nesting gets deeper... 

then nice do:

packet.format2 *formatptr = &apacketpointer->format2; formatptr->value2; // etc 

again, refactoring bunch of preceding typedefs solve problem, lose nice namespacing effect of nested dotted references.

for work in c90, can use macro modeled on toolchain's offsetof() macro:

#define sizeof_field(s,m) (sizeof((((s*)0)->m))) 

adjust accordingly if toolchain's offsetof() macro isn't based on casting 0 pointer structure's type.

when use so:

std::cout << sizeof_field(packet,format1) << std::endl; std::cout << sizeof_field(packet,format2) << std::endl; 

i output:

6 16 

for second question, if you're willing rely on gcc's typeof extension can create similar macro declaring pointers nested anonymous structs:

#define typeof_field(s,m) typeof(((s*)0)->m)  ... typeof_field(packet,format2)* f2 = &foo.format2; 

to honest, find construct pretty ugly, might still better other options have available.

gcc documents "operand of typeof evaluated side effects if , if expression of variably modified type or name of such type", apparent null pointer deference should not result in undefined behavior when variable length array not involved.