Dynamically Sized Structs

[Justin Whear]

On Wed, 16 Apr 2014 23:15:40 +0000, Jeroen Bollen wrote:

> Is it possible to have a structure with a dynamic size? The structure
> would contain an array.
> 
> I know I can use templates, but the size won't be known at compile time.
> I also know I could just put a dynamic array into it, but that way it
> would just be a pointer.
> 
> I know there would be major issues like how to pass the struct to a
> function, as it has an unknown size, but to be quite honest I just want
> pretty code. I'm doing network related operations. I was hoping there'd
> still be a way to do this using templates or so? I just don't want to go
> through the hassle of writing a constructor for it to fill in all the
> fields, and a toByteArray method to convert it back to raw data.
> 
> struct MyStruct {
>          ulong length;
>          ubyte[length] data; // obv won't compile
> }

No, you can't make the structs dynamic.  I had a similar situation 
reading shapefiles a while ago and approached it like this:

struct lengthOf
{
   string fieldName;
}

struct Polygon
{
   ulong numParts;
   @lengthOf("numParts") Part[] parts;

   ulong numPoints;
   @lengthOf("numPoints") Point[] points;
}

T read(T)(...)
    if (/** T is one of your wire-format structs **/)
{
    T ret;
    foreach (I, field; ret.tupleof)
    {
        static if (isDynamicArray!(typeof(field)))
        {
            // use __traits(getAttributes) on the field to get the 
lengthOf attribute
            // mixin a line like this:
            mixin(`field.length = ret.`~lengthOf.fieldName~`;`);
            rawRead(field);

        } else {
            // Assume the field is a primitive and read it
        }
    }
    return ret;
}

Can't find the actual code at the moment, but that's the gist of it.