ref parameter qualifier and static arrays

[anonymous via Digitalmars-d-learn]

On Wednesday 09 September 2015 22:18, Paul wrote:

> 
> Is it possible to call a function like this...
> 
> void foo(ref int[] anArray)
> 
> ...with slices of static arrays? I thought I might be able to use 
> [0..$-1] but to no avail - I get an error like this (which is 
> confusing!):
> 
> (ref int[] anArray) is not callable using argument types (int[])

Slice expressions are not lvalues (can't take their addresses), so you can't 
pass them in ref parameters. You can store the slice in a variable and pass 
that:

----
void f(ref int[] x) {}
int[3] a;
version(none) f(a[]); /* Nope, a[] is not an lvalue. */
int[] s = a[];
f(s); /* Ok, s is an lvalue. */
----

But if you want to pass slice expressions, then you probably don't need that 
ref.

When you pass a slice (without ref), what's actually passed is a pointer and 
length. The contents are not copied. That means, when you alter an array 
element, the change will be done the original, even without ref:

----
void f(int[] x) {x[0] = 1;}
int[3] a = [0, 0, 0];
f(a[]);
assert(a[0] == 1); /* passes */
----

ref would allow you to resize the original dynamic array:

----
void f(ref int[] x) {x ~= 4;}
int[] a = [1, 2, 3];
f(a);
assert(a.length == 4); /* passes */
----

But there's no point in doing that to temporaries like slice expressions.