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.

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 ...

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"));
}

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.)

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;
}

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