* Access conversions
@ 1998-10-12 0:00 Simon Wright
1998-10-12 0:00 ` Matthew Heaney
1998-10-12 0:00 ` Tucker Taft
0 siblings, 2 replies; 4+ messages in thread
From: Simon Wright @ 1998-10-12 0:00 UTC (permalink / raw)
I need to write a function
function Is_Member (G : Graph'Class; V : Vertex'Class) return Boolean;
where there is a type
type Graph_Ptr is access all Graph'Class;
and a Vertex has a component Rep with a component Enclosing of type
Graph_Ptr which accesses its enclosing Graph.
In GNAT I can say in Is_Member
return V.Rep.Enclosing = G'Unrestricted_Access;
but of course this isn't portable.
If I say
function To_Ptr is new Ada.Unchecked_Conversion (System.Address,
Graph_Ptr);
and then in Is_Member
return V.Rep.Enclosing = To_Ptr (G'Address);
which appears to work as expected in GNAT and ObjectAda 7.1, am I in
fact riding for a fall?
-Simon
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Access conversions
1998-10-12 0:00 Access conversions Simon Wright
@ 1998-10-12 0:00 ` Matthew Heaney
1998-10-12 0:00 ` Tucker Taft
1 sibling, 0 replies; 4+ messages in thread
From: Matthew Heaney @ 1998-10-12 0:00 UTC (permalink / raw)
Simon Wright <simon@pogner.demon.co.uk> writes:
> I need to write a function
>
> function Is_Member (G : Graph'Class; V : Vertex'Class) return Boolean;
>
> where there is a type
>
> type Graph_Ptr is access all Graph'Class;
>
> and a Vertex has a component Rep with a component Enclosing of type
> Graph_Ptr which accesses its enclosing Graph.
>
> In GNAT I can say in Is_Member
>
> return V.Rep.Enclosing = G'Unrestricted_Access;
>
> but of course this isn't portable.
>
> If I say
>
> function To_Ptr is new Ada.Unchecked_Conversion (System.Address,
> Graph_Ptr);
>
> and then in Is_Member
>
> return V.Rep.Enclosing = To_Ptr (G'Address);
>
> which appears to work as expected in GNAT and ObjectAda 7.1, am I in
> fact riding for a fall?
>
> -Simon
Instead of converting the address to an access type, and then comparing
access types, why not compare addresses directly:
return V.Rep.Enclosing.all'Address = G'Address;
Isn't this portable?
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Access conversions
1998-10-12 0:00 Access conversions Simon Wright
1998-10-12 0:00 ` Matthew Heaney
@ 1998-10-12 0:00 ` Tucker Taft
1998-10-13 0:00 ` Simon Wright
1 sibling, 1 reply; 4+ messages in thread
From: Tucker Taft @ 1998-10-12 0:00 UTC (permalink / raw)
Simon Wright (simon@pogner.demon.co.uk) wrote:
: I need to write a function
: function Is_Member (G : Graph'Class; V : Vertex'Class) return Boolean;
: where there is a type
: type Graph_Ptr is access all Graph'Class;
: and a Vertex has a component Rep with a component Enclosing of type
: Graph_Ptr which accesses its enclosing Graph.
: In GNAT I can say in Is_Member
: return V.Rep.Enclosing = G'Unrestricted_Access;
: but of course this isn't portable.
If, inside Is_Member, you defined:
type Graph_Const_Ptr is access constant Graph'Class;
then you could portably say:
return Graph_Const_Ptr(V.Rep.Enclosing) = G'Access;
because all tagged formal parameters are considered aliased.
: If I say
: function To_Ptr is new Ada.Unchecked_Conversion (System.Address,
: Graph_Ptr);
: and then in Is_Member
: return V.Rep.Enclosing = To_Ptr (G'Address);
: which appears to work as expected in GNAT and ObjectAda 7.1, am I in
: fact riding for a fall?
The other suggestion of using 'Address on both sides of the
equality operator seems cleaner, though as mentioned above,
G'Access for Graph_Const_Ptr is probably the cleanest, as this survives
the case when V.Rep.Enclosing is null, whereas the .all'Address solution
would raise Constraint_Error.
: -Simon
--
-Tucker Taft stt@inmet.com http://www.inmet.com/~stt/
Intermetrics, Inc. Burlington, MA USA
An AverStar Company
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Access conversions
1998-10-12 0:00 ` Tucker Taft
@ 1998-10-13 0:00 ` Simon Wright
0 siblings, 0 replies; 4+ messages in thread
From: Simon Wright @ 1998-10-13 0:00 UTC (permalink / raw)
stt@houdini.camb.inmet.com (Tucker Taft) writes:
> Simon Wright (simon@pogner.demon.co.uk) wrote:
> : I need to write a function
> : function Is_Member (G : Graph'Class; V : Vertex'Class) return Boolean;
[...]
> : return V.Rep.Enclosing = G'Unrestricted_Access;
>
> : but of course this isn't portable.
>
> If, inside Is_Member, you defined:
>
> type Graph_Const_Ptr is access constant Graph'Class;
>
> then you could portably say:
>
> return Graph_Const_Ptr(V.Rep.Enclosing) = G'Access;
>
> because all tagged formal parameters are considered aliased.
And this also works if the parameter is just G : Graph. Cool.
I must say, this area is a touch complex; I tried
return V.Rep.Enclosing = G'Access;
bc-graphs.adb:191:32: object has deeper accessibility level than access type
return V.Rep.Enclosing = Graph_Ptr (G'Access);
bc-graphs.adb:191:32: argument of conversion cannot be access
bc-graphs.adb:191:44: operand type has deeper accessibility level than target
return V.Rep.Enclosing.all'Access = G'Access;
bc-graphs.adb:191:41: two access attributes cannot be compared directly
bc-graphs.adb:191:41: they must be converted to an explicit type for comparison
so as you can see I was scratching around rather reprehensibly.
The right way to do things is usually obviously right, though not
always obvious.
> : If I say
> : function To_Ptr is new Ada.Unchecked_Conversion (System.Address,
> : Graph_Ptr);
> : and then in Is_Member
> : return V.Rep.Enclosing = To_Ptr (G'Address);
>
> : which appears to work as expected in GNAT and ObjectAda 7.1, am I in
> : fact riding for a fall?
>
> The other suggestion of using 'Address on both sides of the
> equality operator seems cleaner, though as mentioned above,
> G'Access for Graph_Const_Ptr is probably the cleanest, as this survives
> the case when V.Rep.Enclosing is null, whereas the .all'Address solution
> would raise Constraint_Error.
Well, V.Rep.Enclosing _shouldn't_ ever be null; there should perhaps
be a validation in there ..
> -Tucker Taft stt@inmet.com http://www.inmet.com/~stt/
> Intermetrics, Inc. Burlington, MA USA
> An AverStar Company
-Simon
^ permalink raw reply [flat|nested] 4+ messages in thread
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