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digitalmars.D.learn - cast(A)b is not an lvalue

reply "Namespace" <rswhite4 googlemail.com> writes:
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?
Dec 26 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
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
Dec 26 2012
parent reply "Namespace" <rswhite4 googlemail.com> writes:
 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.
Dec 26 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
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
Dec 26 2012
next sibling parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
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...
}
Dec 26 2012
parent "monarch_dodra" <monarchdodra gmail.com> writes:
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 :)
Dec 26 2012
prev sibling parent reply "Maxim Fomin" <maxim maxim-fomin.ru> writes:
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.
Dec 26 2012
next sibling parent "Namespace" <rswhite4 googlemail.com> writes:
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.
Dec 26 2012
prev sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
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.
Dec 26 2012