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
