Re: Trouble checking for null-ness
On 07/25/2016 05:47 PM, Adam D. Ruppe wrote: > On Monday, 25 July 2016 at 13:09:22 UTC, Bahman Movaqar wrote: >> From what I could gather, it's not possible to check for `null` at >> runtime for reference based types. Am I right? > > No, it is only possible to check for null for reference based types. But > map's result is not a reference based type. Oh, I see now. Thanks. -- Bahman
Re: Trouble checking for null-ness
On Monday, 25 July 2016 at 13:09:22 UTC, Bahman Movaqar wrote: From what I could gather, it's not possible to check for `null` at runtime for reference based types. Am I right? No, it is only possible to check for null for reference based types. But map's result is not a reference based type.
Re: Trouble checking for null-ness
On 07/25/2016 05:07 PM, Bahman Movaqar wrote:
> Suppose I have the following function:
>
> public auto max(alias comp, Range)(Range r)
> in {
> assert(r !is null && !r.empty);
> }
> body {
> // ...
> }
>
> When the function after a series of chained `map` operations, I get the
> following error:
>
> Error: incompatible types for ((r) !is (null)):
> 'MapResult!(__lambda2, SInvoiceLine[])' and 'typeof(null)'
>
> Of course if I remove `r !is null` from the `in` block, everything will
> work. But I'm curious; how can I check for a `null` in this case?
Thank you people for the answers.
>From what I could gather, it's not possible to check for `null` at
runtime for reference based types. Am I right?
--
Bahman
Re: Trouble checking for null-ness
On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote:
Suppose I have the following function:
public auto max(alias comp, Range)(Range r)
in {
assert(r !is null && !r.empty);
}
body {
// ...
}
When the function after a series of chained `map` operations, I
get the following error:
Error: incompatible types for ((r) !is (null)):
'MapResult!(__lambda2, SInvoiceLine[])' and 'typeof(null)'
Of course if I remove `r !is null` from the `in` block,
everything will work. But I'm curious; how can I check for a
`null` in this case?
In the general case, the vast majority of ranges you work with
will be value types. In the rare case where it's a reference
type, you can use static if to specialize the assert. Two
possibilities have already been suggested in this thread, but
given that ranges are nearly always going to be a struct or a
class, then you might do this inside the template:
static if(is(Range == class)) { // check for null }
You may also want to add a constraint:
import std.range : isInputRange;
public auto max(alias comp, Range)(Range r)
if(isInputRange!Range) {
static if(is(Range == class)) assert(r !is null && !r.empty);
else assert(!r.empty);
}
Re: Trouble checking for null-ness
On Monday, 25 July 2016 at 12:47:25 UTC, Cauterite wrote: (!__traits(compiles, r is null) || r !is null) && !r.empty Ah, whoops that's wrong, looks like ketmar had the right idea.
Re: Trouble checking for null-ness
On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote: But I'm curious; how can I check for a `null` in this case? Well, if you're happy with assertion failure by access violation, you may not even need to check for null, because generally if you try to call .empty on a null pointer you'll get an access violation (killing two birds with one stone). Otherwise you could try (!__traits(compiles, r is null) || r !is null) && !r.empty
Re: Trouble checking for null-ness
On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote: Error: incompatible types for ((r) !is (null)): 'MapResult!(__lambda2, SInvoiceLine[])' and 'typeof(null)' Of course if I remove `r !is null` from the `in` block, everything will work. But I'm curious; how can I check for a `null` in this case? You can't. null is not a valid value for the return type of map.
Re: Trouble checking for null-ness
static if (is(typeof(r is null))) { ...you can do your assert
here... }
Trouble checking for null-ness
Suppose I have the following function:
public auto max(alias comp, Range)(Range r)
in {
assert(r !is null && !r.empty);
}
body {
// ...
}
When the function after a series of chained `map` operations, I get the
following error:
Error: incompatible types for ((r) !is (null)):
'MapResult!(__lambda2, SInvoiceLine[])' and 'typeof(null)'
Of course if I remove `r !is null` from the `in` block, everything will
work. But I'm curious; how can I check for a `null` in this case?
Thanks,
--
Bahman
