On Monday, 25 November 2019 at 09:00:49 UTC, Dukc wrote:
> On Monday, 25 November 2019 at 03:07:08 UTC, Fanda Vacek wrote:
>> Is this preferred design pattern?
>>
>> ```
>> int main()
>> {
>> 	int a = 1;
>> 	//ref int b = a; // Error: variable `tst_ref.main.b` only parameters or `foreach` declarations can be `ref`
>> 	ref int b() { return a; }
>> 	b = 2;
>> 	assert(a == 2);
>> 	return 0;
>> }
>> ```
>>
>> Fanda
>
> It's okay, but I'd prefer an alias, because your snippet uses the heap needlessly (it puts variable a into heap to make sure there will be no stack corruption if you pass a pointer to function b() outside the main() function)

Yes alias can help, but my original need was like

void main() @safe
{
	int[] a = [1, 2];
	alias head = a[0];
	head = 2;
	assert(a[0] == 2);
}

there the alias doesn't work

Fanda

On Mon, Nov 25, 2019 at 08:39:08PM +0000, Fanda Vacek via Digitalmars-d-learn wrote:
> On Monday, 25 November 2019 at 08:32:53 UTC, H. S. Teoh wrote:
> > On Mon, Nov 25, 2019 at 08:07:50AM +0000, Fanda Vacek via Digitalmars-d-learn wrote: [...]
> > > But anyway, pointers are not allowed in @safe code, so this is not always solution.
> > [...]
> > 
> > This is incorrect. Pointers *are* allowed in @safe code. Pointer *arithmetic* is not allowed.
[...]
> void main() @safe
> {
> 	int a = 1;
> 	int *b = &a;
> 	*b = 2;
> 	assert(a == 2);
> }
> 
> does not compile for me with Error: cannot take address of local `a` in `@safe` function `main`
> 
> so there must be more restrictions in @safe code

Correct, taking the address of a local variable is not allowed because of the possibility of a dangling pointer if the pointer leaks past the end of the variable's scope. With -dip1000, though, it should be allowed as long as you're not leaking the pointer past the lifetime of the local.

Taking the address of a global is perfectly fine in @safe code, as is using a pointer to a heap-allocated object (as long as no pointer arithmetic is involved).


T

-- 
If you're not part of the solution, you're part of the precipitate.