On 10/16/20 4:44 PM, ag0aep6g wrote:
On 16.10.20 22:32, Per Nordlöw wrote:
Why is `T.sizeof` 12 instead of 8 when `U.sizeof` is 8 in the
following example?
struct S
{
int i;
bool b;
}
struct T
{
S s;
char c;
}
struct U
{
int i;
bool b;
char c;
}
?
S.sizeof: 4 bytes for the int + 1 byte for the bool + 3 bytes padding so
that the int is aligned = 8 bytes.
T.sizeof: 8 bytes for the S + 1 byte for the char + 3 bytes padding so
that the S is aligned = 12 bytes.
U.sizeof: 4 bytes for the int + 1 byte for the bool + 1 byte for the
char + 2 bytes padding so that the int is aligned = 8 bytes.
To further explain this -- the padding is added so things like pointer
arithmetic on an array work.
For example, if you have a T* t, and you say t += 1, you want it to go
to the next T, not to a misaligned spot.
You can also override this with align keyword. But I don't recommended
this unless you know what you are doing. Misaligned reads/writes are
different on different architectures, but even if they work and don't
crash your program, they are going to be slower.
https://dlang.org/spec/attribute.html#align
-Steve
