Re: Labels in struct
On Saturday, 31 January 2015 at 16:04:36 UTC, tcak wrote:
I do not have a big example in the end to show, but is there
any way to put labels into struct definitions?
struct CommunicationMessage{
char[2] signature;
mainData:
int info1;
int info2;
extraData:
ushort checksum;
content:
}
Example I defined something like above. I am using it as a base
structure, and don't know how long the content of message will
be. But I know that it will be at the end. I could use that
content label to find out about end of struct. But
unfortunately, it doesn't seem like it is supported.
I could say void* endOfStruct = struct + sizeof(struct), but
then struct wouldn't be self explanatory with that content
label at the end.
... **without** that content label ...
Re: Labels in struct
You could use a UDA like that, though getting the offset will be
a bit tricky.
The idea is to use some uda type, even plain string is good
enough, then use the __traits to find the first appearance of
that UDA in the object and return that offset.
Keep in mind that the @uda: syntax applies it to ALL following
members, it isn't really a label, but if we only look at the
first time it shows up we can kinda pretend it is one.
Here's an example:
struct Foo {
int b;
@(label_one): // offset 4
int c;
int d;
@(label_two): // offset 12
int e;
}
// get the offset of the label on a type, see below for usage
size_t offsetOf(T, string label)() {
foreach(memberName; __traits(allMembers, T)) {
foreach(attribute; __traits(getAttributes, __traits(getMember,
T, memberName))) {
static if(is(typeof(attribute) == string) attribute ==
label)
return __traits(getMember, T,
memberName).offsetof;
}
}
assert(0, no such label);
}
void main() {
// gives what we expect
pragma(msg, offsetOf!(Foo, label_one));
pragma(msg, offsetOf!(Foo, label_two));
}
Re: Labels in struct
Oh this hack also won't work with a label at the very end. You could use sizeof for that though (however that would include padding bytes. Perhaps last member's offsetof plus last member's sizeof is a bit more accurate.)
Re: Labels in struct
On 01/31/15 17:04, tcak via Digitalmars-d-learn wrote:
struct CommunicationMessage{
[...]
content:
}
Example I defined something like above. I am using it as a base structure,
and don't know how long the content of message will be. But I know that it
will be at the end. I could use that content label to find out about end of
struct. But unfortunately, it doesn't seem like it is supported.
I could say void* endOfStruct = struct + sizeof(struct), but then struct
wouldn't be self explanatory with that content label at the end.
The traditional way (ie C-like) would be
ubyte[0] content; // zero-sized; use casts etc to access data.
as the last member. D supports that too, and just like
many other D features it works for ~80% of cases. IOW
you should be able to get it to work, but you might run
into problems if you need to access/manipulate such types.
artur
Re: Labels in struct
On 31.01.15 17:04, tcak wrote:
I do not have a big example in the end to show, but is there any way to
put labels into struct definitions?
struct CommunicationMessage{
char[2] signature;
mainData:
int info1;
int info2;
extraData:
ushort checksum;
content:
}
Members of type struct are actualy labels.
struct CommunicationMessage{
struct Md {
int info1;
int info2;
};
struct Ed {
ushort checksum;
}
struct Cnt {
}
char[2] signature;
Md mainData;
Ed extraData;
Cnt content;
}
