Difference between concatenation and appendation

["岩倉 澪" via Digitalmars-d-learn]

On Monday, 26 January 2015 at 01:17:17 UTC, WhatMeWorry wrote:
> Ok, I just made up that word. But what is the difference 
> between appending and concatenating?  Page 100 of TPDL says 
> "The result of the concatenation is a new array..." and the 
> section on appending talks about possibly needing expansion and 
> reallocation of memory.
>
> But I still don't feel like I have a grasp on the subtleties 
> between them. Can someone give a short and sweet "rule of 
> thumb"?
>
> It might be so obvious that I'll regret posting this.
>
> Thanks.

I'm no expert, so take what I say with a grain of salt. That 
said, here is my understanding:

When you append to an array with ~=, it attempts to reallocate 
the array in-place, meaning it allocates on top of the already 
used space, but grabs some more space past the end of the array. 
If there isn't enough space after the array then obviously it 
can't do that, so it allocates memory somewhere else that it can 
fit and then it copies the contents of the array to the new 
location. If you were to do myArray = myArray ~ moreStuff; I 
assume this is no different from ~=. Conceptually ~= is just 
syntactic sugar in the same way that += or -= is, you are just 
doing a concatenation and then updating the array to point to the 
new result. The fact that it can reallocate in place if there is 
enough space is just like an optimization, in my mind.