Thread overview
Fixed length vs dynamic arrays
May 06

Is a fixed length array created on the heap and a dynamic array on the stack ? And is this important with relation to the garbage collector ?

May 06

On Thursday, 6 May 2021 at 22:35:38 UTC, Alain De Vos wrote:

>

Is a fixed length array created on the heap and a dynamic array on the stack ? And is this important with relation to the garbage collector ?

Static arrays (T[N]) are value types, like structs. If you declare them as local variables, they're allocated on the stack.

Dynamic arrays (T[]), also known as slices, are reference types, consisting of a pointer and a length. If you declare them as local variables, the pointer and length will be stored on the stack. The data they point to, however, may be stored on either the stack or the heap.

For more information about slices in D, I highly recommend this excellent article by Steven Schveighoffer:

https://dlang.org/articles/d-array-article.html

May 07

Following code compiles without errors, only errors during runtime

int main(){
	int[2]  a=new int[2];
	int[]   b=new int[0];
	int[2]  c=[1,2];
	a=a~3;
	b=b~3;
	c=c~3;
	c=b;
	b=a;
	a=c;
return 0;
}
May 08

On Friday, 7 May 2021 at 22:21:09 UTC, Alain De Vos wrote:

>

Following code compiles without errors, only errors during runtime

int main(){
	int[2]  a=new int[2];
	int[]   b=new int[0];
	int[2]  c=[1,2];
	a=a~3;
	b=b~3;
	c=c~3;
	c=b;
	b=a;
	a=c;
return 0;
}

The result of a ~ expression is always a newly-allocated dynamic array on the GC heap. [1]

When you assign a dynamic array to a static array, the elements from the dynamic array on the right-hand side are copied into the static array on the left-hand side. [2] So the statement

a = a ~ 3;

...is essentially shorthand for

auto _tmp = a ~ 3;
foreach (i; 0 .. _tmp.length)
    a[i] = _tmp[i];

Because a has length 2 and a ~ 3 has length 3, this results in a RangeViolation at runtime when attempting to copy _tmp[2] into a[2].

[1] https://dlang.org/spec/expression.html#cat_expressions
[2] https://dlang.org/spec/arrays.html#array-copying