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digitalmars.D - Casts

reply bearophile <bearophileHUGS lycos.com> writes:

http://rusek.org/stefan/default.aspx/2008/10/22/the-3-cast-operators-in-c/73/


Do you see this as safer than the current D1 cast?




if (cast(Foo)o !is null) { ...
That sometimes I write:
if (IsInstance!(Foo)(o)) { ...


something = o as Foo ?? new Foo(0);

Becomes in D:
something = cast(Foo)o;
if (something is null) something = new Foo(0);

Because this shorter code requires two casts, it's slower:
something = cast(Foo)o is null ? new Foo(0) : cast(Foo)o;

I think in D may be useful to split the current cast() into two different
syntaxes, one similar the current dynamc cast, and the other that performs a
non-aliasing re-interepretation of the given bitpattern (the kind of conversion
unions are often used for in C). I find those two cases quite different, but I
confuse them in the current D1, so I think they may deserve two different
syntaxes. 

(This may look like an increase of the complexity of D, but when two (or more)
usages are conflated into a single syntax (or one common usage doesn't have a
hansy syntax) then the complexity of the language seems actually higher).

--------------

Sometimes you need to test a given object to many classes:

if (cast(Foo1)obj !is null) {
    ...
} else if (cast(Foo2)obj !is null) {
    ...
} else if (cast(Foo3)obj !is null) {
    ...
} else if (cast(Foo4)obj !is null) {
    ...
} else {
    ...
}

If such idioms are used often enough (in my code I don't use them often) then
something shorter and/or faster may be adopted.

Bye,
bearophile
Oct 22 2008
parent reply "Denis Koroskin" <2korden gmail.com> writes:
On Wed, 22 Oct 2008 22:39:10 +0400, bearophile <bearophileHUGS lycos.com>  
wrote:



 http://rusek.org/stefan/default.aspx/2008/10/22/the-3-cast-operators-in-c/73/


 done. Do you see this as safer than the current D1 cast?




 if (cast(Foo)o !is null) { ...
 That sometimes I write:
 if (IsInstance!(Foo)(o)) { ...


 something = o as Foo ?? new Foo(0);

 Becomes in D:
 something = cast(Foo)o;
 if (something is null) something = new Foo(0);

 Because this shorter code requires two casts, it's slower:
 something = cast(Foo)o is null ? new Foo(0) : cast(Foo)o;

 I think in D may be useful to split the current cast() into two  
 different syntaxes, one similar the current dynamc cast, and the other  
 that performs a non-aliasing re-interepretation of the given bitpattern  
 (the kind of conversion unions are often used for in C). I find those  
 two cases quite different, but I confuse them in the current D1, so I  
 think they may deserve two different syntaxes.

 (This may look like an increase of the complexity of D, but when two (or  
 more) usages are conflated into a single syntax (or one common usage  
 doesn't have a hansy syntax) then the complexity of the language seems  
 actually higher).

 --------------

 Sometimes you need to test a given object to many classes:

 if (cast(Foo1)obj !is null) {
     ...
 } else if (cast(Foo2)obj !is null) {
     ...
 } else if (cast(Foo3)obj !is null) {
     ...
 } else if (cast(Foo4)obj !is null) {
     ...
 } else {
     ...
 }

 If such idioms are used often enough (in my code I don't use them often)  
 then something shorter and/or faster may be adopted.

 Bye,
 bearophile


You can use the following short-cut to avoid double casting:

if (auto foo = cast(Foo)obj) {
     ...
} else if (auto bar = cast(Bar)bar) {
     ...
} else {
     ...
}
Oct 22 2008
parent reply timotheecour <timothee.cour2 gmail.com> writes:
On Wednesday, 22 October 2008 at 18:43:15 UTC, Denis Koroskin 
wrote:

 On Wed, 22 Oct 2008 22:39:10 +0400, bearophile 
 <bearophileHUGS lycos.com> wrote:


 [...]


 You can use the following short-cut to avoid double casting:

 if (auto foo = cast(Foo)obj) {
     ...
 } else if (auto bar = cast(Bar)bar) {
     ...
 } else {
     ...
 }


doesn't work with extern(C++) classes see 
https://forum.dlang.org/post/nolylatfyjktnvweyxlu forum.dlang.org
Feb 06 2018
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/6/18 4:16 PM, timotheecour wrote:

 On Wednesday, 22 October 2008 at 18:43:15 UTC, Denis Koroskin wrote:

 On Wed, 22 Oct 2008 22:39:10 +0400, bearophile 
 <bearophileHUGS lycos.com> wrote:


 [...]


 You can use the following short-cut to avoid double casting:

 if (auto foo = cast(Foo)obj) {
     ...
 } else if (auto bar = cast(Bar)bar) {
     ...
 } else {
     ...
 }


 
 doesn't work with extern(C++) classes see 
 https://forum.dlang.org/post/nolylatfyjktnvweyxlu forum.dlang.org


Thimotheecour, I'm not sure how you find these threads. This one is 
almost 10 years old! Back then, C++ class integration wasn't even a 
twinkling in Walter's eye ;)

-Steve
Feb 06 2018
parent reply Jonathan Marler <johnnymarler gmail.com> writes:
On Tuesday, 6 February 2018 at 21:35:11 UTC, Steven Schveighoffer 
wrote:

 On 2/6/18 4:16 PM, timotheecour wrote:

 On Wednesday, 22 October 2008 at 18:43:15 UTC, Denis Koroskin 
 wrote:

 On Wed, 22 Oct 2008 22:39:10 +0400, bearophile 
 <bearophileHUGS lycos.com> wrote:


 [...]


 You can use the following short-cut to avoid double casting:

 if (auto foo = cast(Foo)obj) {
     ...
 } else if (auto bar = cast(Bar)bar) {
     ...
 } else {
     ...
 }


 
 doesn't work with extern(C++) classes see 
 https://forum.dlang.org/post/nolylatfyjktnvweyxlu forum.dlang.org


 Thimotheecour, I'm not sure how you find these threads. This 
 one is almost 10 years old! Back then, C++ class integration 
 wasn't even a twinkling in Walter's eye ;)

 -Steve


Lol, I did the same thing once...I think I may have clicked a 
link on an old thread, and when I went back to the "page view" it 
must have gone back to the "page view" where the original thread 
was listed.   I saw an interesting thread an responded and then 
someone let me know it was 7 years old...I had no idea!!!
Feb 06 2018
parent reply jmh530 <john.michael.hall gmail.com> writes:
On Tuesday, 6 February 2018 at 21:41:14 UTC, Jonathan Marler 
wrote:

 Lol, I did the same thing once...I think I may have clicked a 
 link on an old thread, and when I went back to the "page view" 
 it must have gone back to the "page view" where the original 
 thread was listed.   I saw an interesting thread an responded 
 and then someone let me know it was 7 years old...I had no 
 idea!!!


Link didn't work for me either and didn't realize it until I 
checked the date. Was able to use the internet wayback machine to 
get a snap from like 2008.
Feb 06 2018
parent Timothee Cour <thelastmammoth gmail.com> writes:
well even old threads are useful to update when there's new
information ; because they show up in search results so good to keep
answers up to date (and provide link to newer info)


On Tue, Feb 6, 2018 at 1:44 PM, jmh530 via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On Tuesday, 6 February 2018 at 21:41:14 UTC, Jonathan Marler wrote:

 Lol, I did the same thing once...I think I may have clicked a link on an
 old thread, and when I went back to the "page view" it must have gone back
 to the "page view" where the original thread was listed.   I saw an
 interesting thread an responded and then someone let me know it was 7 years
 old...I had no idea!!!



 Link didn't work for me either and didn't realize it until I checked the
 date. Was able to use the internet wayback machine to get a snap from like
 2008.
Feb 06 2018