• "Jarrett Billingsley" <kb3ctd2 yahoo.com> Jul 04 2004
• Mike Parker <aldacron71 yahoo.com> Jul 04 2004
• Andy Friesen <andy ikagames.com> Jul 04 2004
• Norbert Nemec <Norbert.Nemec gmx.de> Jul 05 2004
• Arcane Jill <Arcane_member pathlink.com> Jul 05 2004
• "Walter" <newshound digitalmars.com> Jul 05 2004
• Daniel Horn <hellcatv hotmail.com> Jul 05 2004
• "Walter" <newshound digitalmars.com> Jul 05 2004

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

i'm liking D, but i came across this .... ugliness.

class A
{
 int x;
}

class B
{
 int y;
}

int main()
{
 A a=new A;
 B b=cast(B) a;
 return 0;
}

trying to access members of b will cause a memory access violation.

why am i allowed to do this?

Mike Parker <aldacron71 yahoo.com> writes:

Jarrett Billingsley wrote:
 i'm liking D, but i came across this .... ugliness.
 
 class A
 {
  int x;
 }
 
 class B
 {
  int y;
 }
 
 int main()
 {
  A a=new A;
  B b=cast(B) a;
  return 0;
 }
 
 trying to access members of b will cause a memory access violation.
 
 why am i allowed to do this?
 
 


error with a simple test:

if(b === null)
	printf("B is null");

or alternatively use the 'is' operator, which is the same as '===':

if(b is null)
  ...

Andy Friesen <andy ikagames.com> writes:

Mike Parker wrote:
 When a cast fails, the result is null. So in this case you can catch the 
 error with a simple test:
 
 if(b === null)
     printf("B is null");
 
 or alternatively use the 'is' operator, which is the same as '===':
 
 if(b is null)
  ...


Right, but it's pretty strange that it's not a compile-time error to 
cast an A to a B, even though B does not inherit A.  The cast *has* to 
fail, so why is it even allowed?

  -- andy

Norbert Nemec <Norbert.Nemec gmx.de> writes:

Andy Friesen wrote:

 Mike Parker wrote:
 When a cast fails, the result is null. So in this case you can catch the
 error with a simple test:
 
 if(b === null)
     printf("B is null");
 
 or alternatively use the 'is' operator, which is the same as '===':
 
 if(b is null)
  ...


 Right, but it's pretty strange that it's not a compile-time error to
 cast an A to a B, even though B does not inherit A.  The cast *has* to
 fail, so why is it even allowed?


True. The specifications are not very specific in this point (Heyheyhey,
what a pun!!!):

"D differs from C/C++ in another aspect of casts. Any casting of a class
reference to a derived class reference is done with a runtime check to make
sure it really is a proper downcast. This means that it is equivalent to
the behavior of the dynamic_cast operator in C++."

A sentence should be added:

"Casts between references to unrelated classes references are illegal."

Arcane Jill <Arcane_member pathlink.com> writes:

In article <ccalsv$c9v$1 digitaldaemon.com>, Mike Parker says...

When a cast fails, the result is null. So in this case you can catch the 
error with a simple test:


But why should we have to add run-time code for something that could be detected
at compile-time? This is a compile-time error. It should be reported at
compile-time.

Arcane Jill

"Walter" <newshound digitalmars.com> writes:

"Arcane Jill" <Arcane_member pathlink.com> wrote in message
news:ccb2t5$10tm$1 digitaldaemon.com...
 In article <ccalsv$c9v$1 digitaldaemon.com>, Mike Parker says...

When a cast fails, the result is null. So in this case you can catch the
error with a simple test:


 But why should we have to add run-time code for something that could be


 at compile-time? This is a compile-time error. It should be reported at
 compile-time.


It's a runtime error because one may be asking this question as part of the
logic of the implementation of a template that selects one of several
alternatives based on the type of a parameter.

Daniel Horn <hellcatv hotmail.com> writes:

Walter wrote:
 "Arcane Jill" <Arcane_member pathlink.com> wrote in message
 news:ccb2t5$10tm$1 digitaldaemon.com...
 
In article <ccalsv$c9v$1 digitaldaemon.com>, Mike Parker says...


When a cast fails, the result is null. So in this case you can catch the
error with a simple test:


But why should we have to add run-time code for something that could be


 detected
 
at compile-time? This is a compile-time error. It should be reported at
compile-time.


 
 It's a runtime error because one may be asking this question as part of the
 logic of the implementation of a template that selects one of several
 alternatives based on the type of a parameter.
 
 


checks :-), no?

"Walter" <newshound digitalmars.com> writes:

"Daniel Horn" <hellcatv hotmail.com> wrote in message
news:ccc6jd$2t30$1 digitaldaemon.com...
 Walter wrote:
 It's a runtime error because one may be asking this question as part of




 logic of the implementation of a template that selects one of several
 alternatives based on the type of a parameter.


 checks :-), no?


Trying to do everything with specialization has been listed as one of the
problems with C++ generic programming by some people who have made their
reputations writing template libraries, so I am inclined to believe them
<g>.