If I don't comment out line 19 I get:
/home/c803/c821.d(19): Error: function c821.foo (ref A a) is not callable using argument types (B)
/home/c803/c821.d(19): Error: cast(A)b is not an lvalue

Code: http://dpaste.dzfl.pl/89f55c62

Should not work all three?

On 12/26/2012 07:37 AM, Namespace wrote:
> If I don't comment out line 19 I get:
> /home/c803/c821.d(19): Error: function c821.foo (ref A a) is not
> callable using argument types (B)
> /home/c803/c821.d(19): Error: cast(A)b is not an lvalue
>
> Code: http://dpaste.dzfl.pl/89f55c62
>
> Should not work all three?

I can answer the question in the subject line without looking at dpaste: Yes, in many cases the result of a cast operation is an rvalue. It is a temporary that is constructed at the spot for that cast operation.

Imagine casting an int to a double. The four bytes of the int is nowhere close to what the bit representation of a double is, so a double is made at the spot.

Ali

> I can answer the question in the subject line without looking at dpaste: Yes, in many cases the result of a cast operation is an rvalue. It is a temporary that is constructed at the spot for that cast operation.
>
> Imagine casting an int to a double. The four bytes of the int is nowhere close to what the bit representation of a double is, so a double is made at the spot.
>
> Ali

My question is: Should not work all three?
IMO: yes.

On 12/26/2012 09:05 AM, Namespace wrote:
>> I can answer the question in the subject line without looking at
>> dpaste: Yes, in many cases the result of a cast operation is an
>> rvalue. It is a temporary that is constructed at the spot for that
>> cast operation.
>>
>> Imagine casting an int to a double. The four bytes of the int is
>> nowhere close to what the bit representation of a double is, so a
>> double is made at the spot.
>>
>> Ali
>
> My question is: Should not work all three?
> IMO: yes.

Here is the code:

import std.stdio;

static if (!is(typeof(writeln)))
	alias writefln writeln;

class A { }
class B : A { }

void foo(ref A a) { }

void main()
{
	A a = new A();
	A ab = new B();
	B b = new B();
	
	foo(a);
	foo(ab);
	foo(b); // < compile error
}

foo() takes a class _variable_ by reference (not a class _object_ by reference). Since b is not an A variable, one is constructed on the spot.

Imagine foo() actually does what its signature suggest:

void foo(ref A a) {
    a = new B();
}

That line above is an attempt to modify the caller's rvalue.

Ali

On Wednesday, 26 December 2012 at 17:13:14 UTC, Ali Çehreli wrote:
> On 12/26/2012 09:05 AM, Namespace wrote:
>>> I can answer the question in the subject line without looking at
>>> dpaste: Yes, in many cases the result of a cast operation is an
>>> rvalue. It is a temporary that is constructed at the spot for that
>>> cast operation.
>>>
>>> Imagine casting an int to a double. The four bytes of the int is
>>> nowhere close to what the bit representation of a double is, so a
>>> double is made at the spot.
>>>
>>> Ali
>>
>> My question is: Should not work all three?
>> IMO: yes.
>
> Here is the code:
>
> import std.stdio;
>
> static if (!is(typeof(writeln)))
> 	alias writefln writeln;
>
> class A { }
> class B : A { }
>
> void foo(ref A a) { }
>
> void main()
> {
> 	A a = new A();
> 	A ab = new B();
> 	B b = new B();
> 	
> 	foo(a);
> 	foo(ab);
> 	foo(b); // < compile error
> }
>
> foo() takes a class _variable_ by reference (not a class _object_ by reference). Since b is not an A variable, one is constructed on the spot.
>
> Imagine foo() actually does what its signature suggest:
>
> void foo(ref A a) {
>     a = new B();
> }
>
> That line above is an attempt to modify the caller's rvalue.
>
> Ali

The example is much better with a "new A();" actually ;)

//----
class A { }
class B : A
{
    void B_method();
}

void foo(ref A a)
{
    a = new A();
}

void main()
{
    B b = new B();
    foo(b);//so now after this, b holds a A object? That would be catastrophic...
    b.B_method(); //Awwww crap...
}

On Wednesday, 26 December 2012 at 19:45:53 UTC, monarch_dodra wrote:
> On Wednesday, 26 December 2012 at 17:13:14 UTC, Ali Çehreli wrote:
>> On 12/26/2012 09:05 AM, Namespace wrote:
>>>> I can answer the question in the subject line without looking at
>>>> dpaste: Yes, in many cases the result of a cast operation is an
>>>> rvalue. It is a temporary that is constructed at the spot for that
>>>> cast operation.
>>>>
>>>> Imagine casting an int to a double. The four bytes of the int is
>>>> nowhere close to what the bit representation of a double is, so a
>>>> double is made at the spot.
>>>>
>>>> Ali
>>>
>>> My question is: Should not work all three?
>>> IMO: yes.
>>
>> Here is the code:
>>
>> import std.stdio;
>>
>> static if (!is(typeof(writeln)))
>> 	alias writefln writeln;
>>
>> class A { }
>> class B : A { }
>>
>> void foo(ref A a) { }
>>
>> void main()
>> {
>> 	A a = new A();
>> 	A ab = new B();
>> 	B b = new B();
>> 	
>> 	foo(a);
>> 	foo(ab);
>> 	foo(b); // < compile error
>> }
>>
>> foo() takes a class _variable_ by reference (not a class _object_ by reference). Since b is not an A variable, one is constructed on the spot.
>>
>> Imagine foo() actually does what its signature suggest:
>>
>> void foo(ref A a) {
>>    a = new B();
>> }
>>
>> That line above is an attempt to modify the caller's rvalue.
>>
>> Ali
>
> The example is much better with a "new A();" actually ;)

Wait, never mind. Your example is better.

I actually fell in the "trap" thinking my variable got modified :)

On Wednesday, 26 December 2012 at 17:13:14 UTC, Ali Çehreli wrote:
> Here is the code:
>
> import std.stdio;
>
> static if (!is(typeof(writeln)))
> 	alias writefln writeln;
>

What does this for? I constantly face in code samples shared in this NG.

On Wednesday, 26 December 2012 at 19:52:21 UTC, Maxim Fomin wrote:
> On Wednesday, 26 December 2012 at 17:13:14 UTC, Ali Çehreli wrote:
>> Here is the code:
>>
>> import std.stdio;
>>
>> static if (!is(typeof(writeln)))
>> 	alias writefln writeln;
>>
>
> What does this for? I constantly face in code samples shared in this NG.

It's automatically generated from Dpaste templates.

@all:
Thanks for the explanation.

On 12/26/12, Maxim Fomin <maxim@maxim-fomin.ru> wrote:
>> static if (!is(typeof(writeln)))
>> 	alias writefln writeln;
>>
> What does this for? I constantly face in code samples shared in this NG.

Probably for D1 compatibility. D1 didn't have writeln.