mccJan 15

I'm writing C++. I have a struct

struct ExampleStruct {
int32_t i;
bool a;
bool b;
}

I know I'm running on a modern CPU such as x86_64 or ARM64, and I know I'm using a modern compiler such as recent clang, recent gcc, or recent msvc.

The byte offsets of "a" and "b", relative to the start of ExampleStruct, are:

4 and 5
30.2%
4 and 8
16.3%
It depends on platform/compiler
44.1%
U gotta use a #pragma
9.4%
Poll ended at .
Show threadJosh Simmons
@mcc int is 4 bytes size, 4 bytes alignment, bools are 1 byte size, 1 byte alignment. the whole struct is 8 bytes size 4 bytes alignment. there's two bytes of padding at the end. mostly this is load bearing abi stuff so it shouldn't really change underneath you, even if it's not strictly specified by the C++ standard (it is specified by the platform ABI).
Jan 15 at 8:32pmPhanpy
Show threadJosh SimmonsJan 15
@mcc e.g. i believe sizeof(bool) can still be something stupid by the C++ law, in which case you might have a fun time. but if you find a platform where `sizeof(bool)` isn't 1 you can throw it into the fire anyway.
Show threadmccJan 15
@dotstdy And "The ABI" in question is what? The Itanium ABI? A coincidental, silent agreement between the Itanium, ARM, and MS ABIs?
Show threadJosh SimmonsJan 15
@mcc depends on the platform, idk which documents you need to look at to find the answers, but yes probably all three of those are important.
Show threadI Can't Believe It's Not Zero!Jan 15

@mcc @dotstdy the x86 psABI and the ARM PCS define the size and alignment of the fields.

(Compiler implementations could chose to ignore them, but then they can’t interoperate with other binaries, so it’s fairly rare and confined to more niche types.)

Show threadbobJan 15
@dotstdy @mcc even if it's probably safe to assume the padding won't change is there any cost to using #pragma pack
Show threadJosh SimmonsJan 15
@bob @mcc yes, that will make it *less* portable, since it'll be misaligned.