Appending to a dynamic array at specific offset

[Joseph Bell]

Hi.

I'm working with D arrays, getting my bearings, and was looking at 
treating a ubtye array as a formatted message of sorts.

Consider the first byte is always a message type header.
The second and third bytes are a length indicator.
The fourth and subsequent bytes are the actual data this message conveys.

If I declare a dynamic array:

ubyte[] msg;

I can easily write

msg ~= TYPE; // TYPE is a ubyte value
msg ~= LEN_HI; // upper byte of 16-bit length
msg ~= LEN_LOW; // lower byte of 16-bit length

At this point I can

msg ~= data; // where data is a ubyte[]

But if I want to change the data, I'd like to be able to just append the 
new data at the offset it needs to go at.

msg[3] ~= data; // doesn't work, msg[3] is a ubyte, can't append ubyte[]
&msg[3] ~= data; // Doesn't work, perhaps trying to write C there

This does work
msg.length = 3;
msg ~= data; // Effectively resize the array back to 3 and then append

I don't know how ineffecient the above approach is.

Also the following doesn't work:

msg[3..$] = data; // Array sizes don't match for copy
msg[3..$] ~= data; // Slice is not a modifiable lvalue

So the upshot:  I have a working solution to the problem, but it looks 
unnatural to me.  Am I missing a magic syntactical expression for this?

Thanks much for any replies,

Joe
Budding D Zealot trying to kick C/C++ to the curb