D's Array length is currently of type size_t, which means on 32 bit it's an uint and on 64 bit an ulong. This is difficult: What if I want to give the length of an array as parameter to some C functions which accepts only an int?
What is the right/safe way to do this? A cast? Or is there something better?
I like to avoid cast's if they aren't necessary.

On Wednesday, 18 September 2013 at 20:46:21 UTC, Namespace wrote:
> D's Array length is currently of type size_t, which means on 32 bit it's an uint and on 64 bit an ulong. This is difficult: What if I want to give the length of an array as parameter to some C functions which accepts only an int?
> What is the right/safe way to do this? A cast? Or is there something better?
> I like to avoid cast's if they aren't necessary.

Perfectly safe way is to do run-time range check followed by a cast. Maybe using a wrapper:

auto shrinkTo(Target, Source)(Source source)
    if (is(Target : Source)) // is implicitly convertable other way around
{
    assert(source <= Target.max);
    static if (is(typeof(Taget.min)))
        assert(source >= Target.min);
    return cast(Target)source;
}

void main()
{
	import std.stdio;
	long a = 50;
	long b = int.max; b++;
	writeln(shrinkTo!int(a));
	writeln(shrinkTo!int(b));
}

On Wed, Sep 18, 2013 at 10:46:18PM +0200, Namespace wrote:
> D's Array length is currently of type size_t, which means on 32 bit
> it's an uint and on 64 bit an ulong. This is difficult: What if I
> want to give the length of an array as parameter to some C functions
> which accepts only an int?
> What is the right/safe way to do this? A cast? Or is there something
> better?
> I like to avoid cast's if they aren't necessary.

If the C function only accepts int, then just use to!int(array.size). If the size overflows int, to() will throw an exception which you can handle. This is probably the best you can do anyway, since if the C function doesn't take anything bigger than int, then there's no way you can pass the real size to it.


T

-- 
PNP = Plug 'N' Pray

On Wednesday, 18 September 2013 at 22:20:45 UTC, H. S. Teoh wrote:
> If the C function only accepts int, then just use to!int(array.size). If
> the size overflows int, to() will throw an exception which you can
> handle. This is probably the best you can do anyway, since if the C
> function doesn't take anything bigger than int, then there's no way you
> can pass the real size to it.

Ah, `to!()` does check valid ranges before conversion? Good to know, thanks.

On Thu, Sep 19, 2013 at 12:52:55AM +0200, Dicebot wrote:
> On Wednesday, 18 September 2013 at 22:20:45 UTC, H. S. Teoh wrote:
> >If the C function only accepts int, then just use to!int(array.size). If the size overflows int, to() will throw an exception which you can handle. This is probably the best you can do anyway, since if the C function doesn't take anything bigger than int, then there's no way you can pass the real size to it.
> 
> Ah, `to!()` does check valid ranges before conversion? Good to know,
> thanks.

Yes, it's handled explicitly by the following overload of toImpl() in
std.conv:

	T toImpl(T, S)(S value)
	    if (!isImplicitlyConvertible!(S, T) &&
		(isNumeric!S || isSomeChar!S || isBoolean!S) &&
		(isNumeric!T || isSomeChar!T || isBoolean!T) && !is(T == enum))
	{
	    ...
	}


T

-- 
Too many people have open minds but closed eyes.

On Wednesday, 18 September 2013 at 22:20:45 UTC, H. S. Teoh wrote:
> On Wed, Sep 18, 2013 at 10:46:18PM +0200, Namespace wrote:
>> D's Array length is currently of type size_t, which means on 32 bit
>> it's an uint and on 64 bit an ulong. This is difficult: What if I
>> want to give the length of an array as parameter to some C functions
>> which accepts only an int?
>> What is the right/safe way to do this? A cast? Or is there something
>> better?
>> I like to avoid cast's if they aren't necessary.
>
> If the C function only accepts int, then just use to!int(array.size). If
> the size overflows int, to() will throw an exception which you can
> handle. This is probably the best you can do anyway, since if the C
> function doesn't take anything bigger than int, then there's no way you
> can pass the real size to it.
>
>
> T
So to!int is safer but slower and a cast would be unsafe but faster?

Namespace:

> So to!int is safer but slower and a cast would be unsafe but faster?

Right.

Bye,
bearophile

On Thursday, 19 September 2013 at 11:10:08 UTC, bearophile wrote:
> Namespace:
>
>> So to!int is safer but slower and a cast would be unsafe but faster?
>
> Right.
>
> Bye,
> bearophile

Thanks!