How array concatenation works... internally

[Steven Schveighoffer]

On 4/23/18 7:15 PM, Dnewbie wrote:
> Hi,
> 
> I'd like to understand how array concatenation works internally, like 
> the example below:
> 
> //DMD64 D Compiler 2.072.2
> import std.stdio;
> void main(){
>      string[] arr;
>      arr.length = 2;
>      arr[0] = "Hello";
>      arr[1] = "World";
>      writeln(arr.length);
>      arr = arr[0..1] ~ "New String" ~ arr[1..2];
>      writeln(arr.length);
>      foreach(string a; arr){
>          writeln(a);
>      }
> }
> http://rextester.com/DDW84343
> 
> The code above prints:
> 2
> 3
> Hello
> New String
> World
> 
> 
> So, It changes the "arr" length and put the "New String" between the 
> other two. It's very fast with some other tests that I made.

What it has done is built a completely new array, with the new 3 
elements. The old array is still there. You can witness this by keeping 
a reference to the old array:

auto arr2 = arr;
arr = arr[0 .. 1] ~ "New String" ~ arr[1 .. 2];
writeln(arr2);
writeln(arr);

> Now I'm curious to know what's happening under the hood. It's related to 
> memcpy?

There's not much copying going on here, each string is stored in the arr 
as a pointer and length pair. So you are just making a copy of those.

> On Phobos "array.d" source I've found this:
> 
>      /// Concatenation with rebinding.
>      void opCatAssign(R)(R another)
>      {
>          auto newThis = this ~ another;
>          move(newThis, this);
>      }

This is different. The builtin arrays are not part of phobos, they are 
defined by the compiler. std.array.Array is a different type.

If you want to know more about the array runtime, I suggest this 
article: https://dlang.org/articles/d-array-article.html

> But now I'm having problem to find how I can reach this "move" function, 
> since I couldn't find any "move" on the "std" folder.

Move is std.algorithm.move: 
https://dlang.org/phobos/std_algorithm_mutation.html#.move

-Steve