> "you're only supposed to use it when you know what you're doing when you
> use it"
> Well, that's kinda the requirement for any feature, isn't it?... :P
> Well, kinda, I do know what you mean. Yes, one needs to properly learn
> how cast() works, but that doesn't mean it would not be better for the
> feature to be designed in a way that is easier to learn (and just as
> powerful to use), or actually, it doesn't mean that even if you do know
> how cast() works, that you are not still likely to make a mistake when
> using it.
> So pretty much I agree with Nick here. I think me and a few others have
> mentioned it in the past, that the way cast works makes it somewhat easy
> to trip over one's foot and cast away something that was not intended.
> This may be a bit more common now and in the future as people use more
> and more of the newer D modifiers (shared, immutable, etc.).

Yes, let's introduce immutable_cast(), const_cast(), shared_cast(), static_cast(), dynamic_cast(),
reinterpret_cast(), sounds like a good idea lol.

== Quote from Mehrdad (wfunction@hotmail.com)'s article
> > "you're only supposed to use it when you know what you're doing when you
> > use it"
> > Well, that's kinda the requirement for any feature, isn't it?... :P
> > Well, kinda, I do know what you mean. Yes, one needs to properly learn
> > how cast() works, but that doesn't mean it would not be better for the
> > feature to be designed in a way that is easier to learn (and just as
> > powerful to use), or actually, it doesn't mean that even if you do know
> > how cast() works, that you are not still likely to make a mistake when
> > using it.
> > So pretty much I agree with Nick here. I think me and a few others have
> > mentioned it in the past, that the way cast works makes it somewhat easy
> > to trip over one's foot and cast away something that was not intended.
> > This may be a bit more common now and in the future as people use more
> > and more of the newer D modifiers (shared, immutable, etc.).
> Yes, let's introduce immutable_cast(), const_cast(), shared_cast(), static_cast(),
dynamic_cast(),
> reinterpret_cast(), sounds like a good idea lol.

Joking aside, I've actually found a few templates to be helpful for casting only one modifier
at a time:
unconst!(T)(auto ref T value)      //Removes const
unshared!(T)(auto ref T value)     //Remove shared
arrcast!(TTo, T)(auto ref T array) //Array cast, keeping const/immutable, doing length
divisibility checks, etc. Name is ugly, I couldn't think of anything better

Should templates like those be included in std.conv or something?

We put a compiler in yo compiler so you can CTFE while you CTFE.

"Bruno Medeiros" <brunodomedeiros+spam@com.gmail> wrote in message news:is8a25$gka$1@digitalmars.com...
> On 02/06/2011 06:08, Nick Sabalausky wrote:
>> "Andrej Mitrovic"<andrej.mitrovich@gmail.com>  wrote in message news:mailman.529.1306975584.14074.digitalmars-d@puremagic.com...
>>> Stroustrup: What happen ?
>>> Meyers: Somebody set up us the bomb.
>>> Spolsky: We get signal.
>>> Stroustrup: What !
>>> Spolsky: Main screen turn on.
>>> Stroustrup: It’s you !!
>>> Bright: How are you gentlemen !!
>>> Bright: All your base are belong to us.
>>> Bright: You are on the way to destruction.
>>> Stroustrup: What you say !!
>>> Bright: You have no chance to survive make your time.
>>> Bright: Ha ha ha ha …
>>>
>>
>> I don't care how old it is in "internet meme" time: I'll never get tired
>> of
>> those classic lines :)  ('course it's much older than even that...)
>>
>>
>
>
> O RLY? :P
>

CAN I HAZ FUN OLD MEMEZ PLZ, K THNX BYE! :)

On 2011-06-02 09:02, Bruno Medeiros wrote:
> On 01/06/2011 22:12, Jonathan M Davis wrote:
> >> It's nice in a way, but it's all based on the way cast handles modifiers.
> >> 
> >> > And I've always been a bit unconfortable with how that's handled (In fact, I was just thinking about this yesterday). Specifically, it seems extremely bad that it's so incredibly easy to accidentaly cast away things like const and immutable:
> >> > 
> >> > For example, if I know I have an array of uint's, and I want to deal with the individual bytes, it's perfectly safe and sensible to cast it to a ubyte[] (a long as you factor in endianness, of course). So, you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened to be immutable, then merely trying to cast the type has inadvertantly cast-away immutable. Not good! Casting away const/immutable really, really should have to be explict.
> >> > 
> >> > Of course, you can probably use some fancy helper templates to make sure you preserve all modifiers. But needing to do so is just asking for mistakes: it seems like a huge violation of "make the right way easy, and the wrong way hard".
> > 
> > You really shouldn't be casting much anyway. It's the sort of feature where you're only supposed to use it when you know what you're doing when you use it. And if there's really any possibility that you're dealing with immutable, perhaps you should be casting to const rather than mutable. Personally, I find how C++ created multiple types of cast (include const_cast)_highly_ annoying and generally useless, and I'm_very_ glad that D didn't do anything of the sort.
> > 
> > - Jonathan M Davis
> 
> "you're only supposed to use it when you know what you're doing when you
> use it"
> Well, that's kinda the requirement for any feature, isn't it?... :P
> Well, kinda, I do know what you mean. Yes, one needs to properly learn
> how cast() works, but that doesn't mean it would not be better for the
> feature to be designed in a way that is easier to learn (and just as
> powerful to use), or actually, it doesn't mean that even if you do know
> how cast() works, that you are not still likely to make a mistake when
> using it.

Really, you should only be using casts when you need a cast, and you should be careful when you use them. So, you have to know what you're doing and why in that particular instance and not use them willy-nilly. In most code, casts should be quite rare. If you're doing a lot of low-level stuff, then they might be more common, but in general, they're a sledgehammer that shouldn't be used except when you really need them.

> So pretty much I agree with Nick here. I think me and a few others have mentioned it in the past, that the way cast works makes it somewhat easy to trip over one's foot and cast away something that was not intended. This may be a bit more common now and in the future as people use more and more of the newer D modifiers (shared, immutable, etc.).

I really don't know what you'd do other than introducing stuff like C++'s const_cast, and I know very few programmers who actually use any of those special casts. In most cases, they just use C-style casts. It just gets too complicated to determine what the correct cast to use in a particular situation is. And C++ is the only language I know of with more than one type of cast. I think that it's _far_ better to have the compiler figure that out for you. Some templates could be created for changing one aspect of type (such as mutability or shared-ness) if people really want that, but since casts should generally be rare in the first place, the effort of getting them right shouldn't be very high anyway. Regardless, I'd _hate_ to see the built-in cast operator altered to be more complicated. I _much_ prefer it to C++'s special casts, and I'm _very_ glad that they were not one of the C++ features to be carried over to D.

- Jonathan M Davis

On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis <jmdavisProg@gmx.com> wrote:

> On 2011-06-02 09:02, Bruno Medeiros wrote:
>> On 01/06/2011 22:12, Jonathan M Davis wrote:
>> >> It's nice in a way, but it's all based on the way cast handles
>> >> modifiers.
>> >>
>> >> > And I've always been a bit unconfortable with how that's handled  
>> (In
>> >> > fact, I was just thinking about this yesterday). Specifically, it
>> >> > seems extremely bad that it's so incredibly easy to accidentaly  
>> cast
>> >> > away things like const and immutable:
>> >> >
>> >> > For example, if I know I have an array of uint's, and I want to  
>> deal
>> >> > with the individual bytes, it's perfectly safe and sensible to cast
>> >> > it to a ubyte[] (a long as you factor in endianness, of course).  
>> So,
>> >> > you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened to
>> >> > be immutable, then merely trying to cast the type has inadvertantly
>> >> > cast-away immutable. Not good! Casting away const/immutable really,
>> >> > really should have to be explict.
>> >> >
>> >> > Of course, you can probably use some fancy helper templates to make
>> >> > sure you preserve all modifiers. But needing to do so is just  
>> asking
>> >> > for mistakes: it seems like a huge violation of "make the right way
>> >> > easy, and the wrong way hard".
>> >
>> > You really shouldn't be casting much anyway. It's the sort of feature
>> > where you're only supposed to use it when you know what you're doing
>> > when you use it. And if there's really any possibility that you're
>> > dealing with immutable, perhaps you should be casting to const rather
>> > than mutable. Personally, I find how C++ created multiple types of  
>> cast
>> > (include const_cast)_highly_ annoying and generally useless, and
>> > I'm_very_ glad that D didn't do anything of the sort.
>> >
>> > - Jonathan M Davis
>>
>> "you're only supposed to use it when you know what you're doing when you
>> use it"
>> Well, that's kinda the requirement for any feature, isn't it?... :P
>> Well, kinda, I do know what you mean. Yes, one needs to properly learn
>> how cast() works, but that doesn't mean it would not be better for the
>> feature to be designed in a way that is easier to learn (and just as
>> powerful to use), or actually, it doesn't mean that even if you do know
>> how cast() works, that you are not still likely to make a mistake when
>> using it.
>
> Really, you should only be using casts when you need a cast, and you should be
> careful when you use them. So, you have to know what you're doing and why in
> that particular instance and not use them willy-nilly. In most code, casts
> should be quite rare. If you're doing a lot of low-level stuff, then they
> might be more common, but in general, they're a sledgehammer that shouldn't be
> used except when you really need them.

Casting is the recommended way to determine if something is actually a derived type:

interface I {}

class C : I {}

void foo(I i) // change this to const(I) and you have a huge const bug!
{
   if(C c = cast(C)i)
   {
      // optimized branch
   }
   else
   {
      // default branch
   }
}

so it's not so easy to say you should "never" cast.  BTW, dcollections uses this quite a bit to support operations between multiple types of collections.

It would be nice if at least dynamic cast (with the added rules to prevent casting away const/immutable/shared) was a different syntax from cast, since it's a completely safe usage of casting (as long as you manually forward modifiers).  It's as safe as to! is when used with the right context.

-Steve

On 2011-06-02 15:13, Steven Schveighoffer wrote:
> On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis <jmdavisProg@gmx.com>
> 
> wrote:
> > On 2011-06-02 09:02, Bruno Medeiros wrote:
> >> On 01/06/2011 22:12, Jonathan M Davis wrote:
> >> >> It's nice in a way, but it's all based on the way cast handles modifiers.
> >> >> 
> >> >> > And I've always been a bit unconfortable with how that's handled
> >> 
> >> (In
> >> 
> >> >> > fact, I was just thinking about this yesterday). Specifically, it seems extremely bad that it's so incredibly easy to accidentaly
> >> 
> >> cast
> >> 
> >> >> > away things like const and immutable:
> >> >> > 
> >> >> > For example, if I know I have an array of uint's, and I want to
> >> 
> >> deal
> >> 
> >> >> > with the individual bytes, it's perfectly safe and sensible to cast it to a ubyte[] (a long as you factor in endianness, of course).
> >> 
> >> So,
> >> 
> >> >> > you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened to be immutable, then merely trying to cast the type has inadvertantly cast-away immutable. Not good! Casting away const/immutable really, really should have to be explict.
> >> >> > 
> >> >> > Of course, you can probably use some fancy helper templates to make sure you preserve all modifiers. But needing to do so is just
> >> 
> >> asking
> >> 
> >> >> > for mistakes: it seems like a huge violation of "make the right way easy, and the wrong way hard".
> >> > 
> >> > You really shouldn't be casting much anyway. It's the sort of feature where you're only supposed to use it when you know what you're doing when you use it. And if there's really any possibility that you're dealing with immutable, perhaps you should be casting to const rather than mutable. Personally, I find how C++ created multiple types of
> >> 
> >> cast
> >> 
> >> > (include const_cast)_highly_ annoying and generally useless, and I'm_very_ glad that D didn't do anything of the sort.
> >> > 
> >> > - Jonathan M Davis
> >> 
> >> "you're only supposed to use it when you know what you're doing when you
> >> use it"
> >> Well, that's kinda the requirement for any feature, isn't it?... :P
> >> Well, kinda, I do know what you mean. Yes, one needs to properly learn
> >> how cast() works, but that doesn't mean it would not be better for the
> >> feature to be designed in a way that is easier to learn (and just as
> >> powerful to use), or actually, it doesn't mean that even if you do know
> >> how cast() works, that you are not still likely to make a mistake when
> >> using it.
> > 
> > Really, you should only be using casts when you need a cast, and you
> > should be
> > careful when you use them. So, you have to know what you're doing and
> > why in
> > that particular instance and not use them willy-nilly. In most code,
> > casts
> > should be quite rare. If you're doing a lot of low-level stuff, then they
> > might be more common, but in general, they're a sledgehammer that
> > shouldn't be
> > used except when you really need them.
> 
> Casting is the recommended way to determine if something is actually a derived type:
> 
> interface I {}
> 
> class C : I {}
> 
> void foo(I i) // change this to const(I) and you have a huge const bug!
> {
> if(C c = cast(C)i)
> {
> // optimized branch
> }
> else
> {
> // default branch
> }
> }
> 
> so it's not so easy to say you should "never" cast. BTW, dcollections uses this quite a bit to support operations between multiple types of collections.
> 
> It would be nice if at least dynamic cast (with the added rules to prevent casting away const/immutable/shared) was a different syntax from cast, since it's a completely safe usage of casting (as long as you manually forward modifiers). It's as safe as to! is when used with the right context.

No. It's not the case that you should _never_ cast, but in most programs, it should still be rare. And yes, casting is the proper way to determine whether an object's type, is derived from a particular class, but it's generally considered bad OO programming to have to ask that sort of question very often. There are a few places that you're forced to (such as opEquals), but in general, it's a symptom of a bad design. Code should generally be written in such a manner that you either know what it's type is or don't care. You shouldn't generally have to ask it. Obviously, there are exceptions, but they should be exceptions rather than the norm. As such, casts should be fairly rare.

Personally, I find the added complexity of C++'s special casts to _far_ outweight what benefit they give you. Yes, having something like Java's instanceof instead of having to cast to check whether an object's type is derived from a particular type would arguably be an improvement, but even it shouldn't be used all that often, and I definitely don't think that creating a set of cast operators for different types of cast would be a good idea at all.

- Jonathan M Davis

On Thu, 02 Jun 2011 18:39:48 -0400, Jonathan M Davis <jmdavisProg@gmx.com> wrote:

> On 2011-06-02 15:13, Steven Schveighoffer wrote:
>> On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis <jmdavisProg@gmx.com>
>>
>> wrote:
>> > On 2011-06-02 09:02, Bruno Medeiros wrote:
>> >> On 01/06/2011 22:12, Jonathan M Davis wrote:
>> >> >> It's nice in a way, but it's all based on the way cast handles
>> >> >> modifiers.
>> >> >>
>> >> >> > And I've always been a bit unconfortable with how that's handled
>> >>
>> >> (In
>> >>
>> >> >> > fact, I was just thinking about this yesterday). Specifically,  
>> it
>> >> >> > seems extremely bad that it's so incredibly easy to accidentaly
>> >>
>> >> cast
>> >>
>> >> >> > away things like const and immutable:
>> >> >> >
>> >> >> > For example, if I know I have an array of uint's, and I want to
>> >>
>> >> deal
>> >>
>> >> >> > with the individual bytes, it's perfectly safe and sensible to  
>> cast
>> >> >> > it to a ubyte[] (a long as you factor in endianness, of course).
>> >>
>> >> So,
>> >>
>> >> >> > you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened  
>> to
>> >> >> > be immutable, then merely trying to cast the type has  
>> inadvertantly
>> >> >> > cast-away immutable. Not good! Casting away const/immutable  
>> really,
>> >> >> > really should have to be explict.
>> >> >> >
>> >> >> > Of course, you can probably use some fancy helper templates to  
>> make
>> >> >> > sure you preserve all modifiers. But needing to do so is just
>> >>
>> >> asking
>> >>
>> >> >> > for mistakes: it seems like a huge violation of "make the right  
>> way
>> >> >> > easy, and the wrong way hard".
>> >> >
>> >> > You really shouldn't be casting much anyway. It's the sort of  
>> feature
>> >> > where you're only supposed to use it when you know what you're  
>> doing
>> >> > when you use it. And if there's really any possibility that you're
>> >> > dealing with immutable, perhaps you should be casting to const  
>> rather
>> >> > than mutable. Personally, I find how C++ created multiple types of
>> >>
>> >> cast
>> >>
>> >> > (include const_cast)_highly_ annoying and generally useless, and
>> >> > I'm_very_ glad that D didn't do anything of the sort.
>> >> >
>> >> > - Jonathan M Davis
>> >>
>> >> "you're only supposed to use it when you know what you're doing when  
>> you
>> >> use it"
>> >> Well, that's kinda the requirement for any feature, isn't it?... :P
>> >> Well, kinda, I do know what you mean. Yes, one needs to properly  
>> learn
>> >> how cast() works, but that doesn't mean it would not be better for  
>> the
>> >> feature to be designed in a way that is easier to learn (and just as
>> >> powerful to use), or actually, it doesn't mean that even if you do  
>> know
>> >> how cast() works, that you are not still likely to make a mistake  
>> when
>> >> using it.
>> >
>> > Really, you should only be using casts when you need a cast, and you
>> > should be
>> > careful when you use them. So, you have to know what you're doing and
>> > why in
>> > that particular instance and not use them willy-nilly. In most code,
>> > casts
>> > should be quite rare. If you're doing a lot of low-level stuff, then  
>> they
>> > might be more common, but in general, they're a sledgehammer that
>> > shouldn't be
>> > used except when you really need them.
>>
>> Casting is the recommended way to determine if something is actually a
>> derived type:
>>
>> interface I {}
>>
>> class C : I {}
>>
>> void foo(I i) // change this to const(I) and you have a huge const bug!
>> {
>> if(C c = cast(C)i)
>> {
>> // optimized branch
>> }
>> else
>> {
>> // default branch
>> }
>> }
>>
>> so it's not so easy to say you should "never" cast. BTW, dcollections
>> uses this quite a bit to support operations between multiple types of
>> collections.
>>
>> It would be nice if at least dynamic cast (with the added rules to prevent
>> casting away const/immutable/shared) was a different syntax from cast,
>> since it's a completely safe usage of casting (as long as you manually
>> forward modifiers). It's as safe as to! is when used with the right
>> context.
>
> No. It's not the case that you should _never_ cast, but in most programs, it
> should still be rare. And yes, casting is the proper way to determine whether
> an object's type, is derived from a particular class, but it's generally
> considered bad OO programming to have to ask that sort of question very often.

> There are a few places that you're forced to (such as opEquals), but in
> general, it's a symptom of a bad design. Code should generally be written in
> such a manner that you either know what it's type is or don't care. You
> shouldn't generally have to ask it. Obviously, there are exceptions, but they
> should be exceptions rather than the norm. As such, casts should be fairly
> rare.

Any place you have double-dispatch, you need to use it in the dispatch function.

> Personally, I find the added complexity of C++'s special casts to _far_
> outweight what benefit they give you. Yes, having something like Java's
> instanceof instead of having to cast to check whether an object's type is
> derived from a particular type would arguably be an improvement, but even it
> shouldn't be used all that often, and I definitely don't think that creating a
> set of cast operators for different types of cast would be a good idea at all.

added complexity?  How so?  There are four types of casts:

static_cast -> cast without discarding const or volatile
const_cast -> cast only removing const or volatile
dynamic_cast -> do a runtime cast to see if an object is actually a derived type
reinterpret_cast -> force everything, and avoid any cast overloads.

In all of these, only const_cast and reinterpret_cast are unsafe.  Each one clearly separates what you wish to do.

this is not complication, this is expressiveness.  Something that D's casting does not have.  D's casting conflates multiple things together, and because of the "I trust you" nature that casts have, you can easily make mistakes without complaint from the compiler.  The fact that a dynamic cast is inherently unsafe in D is unnecessary.  I don't know if it will change, but I can't say it's better than C++.  It can be made better than C++, but I think I'd rather have the four cast types than what D has now.

-Steve

On 2011-06-02 16:22, Steven Schveighoffer wrote:
> On Thu, 02 Jun 2011 18:39:48 -0400, Jonathan M Davis <jmdavisProg@gmx.com>
> 
> wrote:
> > On 2011-06-02 15:13, Steven Schveighoffer wrote:
> >> On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis <jmdavisProg@gmx.com>
> >> 
> >> wrote:
> >> > On 2011-06-02 09:02, Bruno Medeiros wrote:
> >> >> On 01/06/2011 22:12, Jonathan M Davis wrote:
> >> >> >> It's nice in a way, but it's all based on the way cast handles modifiers.
> >> >> >> 
> >> >> >> > And I've always been a bit unconfortable with how that's handled
> >> >> 
> >> >> (In
> >> >> 
> >> >> >> > fact, I was just thinking about this yesterday). Specifically,
> >> 
> >> it
> >> 
> >> >> >> > seems extremely bad that it's so incredibly easy to accidentaly
> >> >> 
> >> >> cast
> >> >> 
> >> >> >> > away things like const and immutable:
> >> >> >> > 
> >> >> >> > For example, if I know I have an array of uint's, and I want to
> >> >> 
> >> >> deal
> >> >> 
> >> >> >> > with the individual bytes, it's perfectly safe and sensible to
> >> 
> >> cast
> >> 
> >> >> >> > it to a ubyte[] (a long as you factor in endianness, of course).
> >> >> 
> >> >> So,
> >> >> 
> >> >> >> > you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened
> >> 
> >> to
> >> 
> >> >> >> > be immutable, then merely trying to cast the type has
> >> 
> >> inadvertantly
> >> 
> >> >> >> > cast-away immutable. Not good! Casting away const/immutable
> >> 
> >> really,
> >> 
> >> >> >> > really should have to be explict.
> >> >> >> > 
> >> >> >> > Of course, you can probably use some fancy helper templates to
> >> 
> >> make
> >> 
> >> >> >> > sure you preserve all modifiers. But needing to do so is just
> >> >> 
> >> >> asking
> >> >> 
> >> >> >> > for mistakes: it seems like a huge violation of "make the right
> >> 
> >> way
> >> 
> >> >> >> > easy, and the wrong way hard".
> >> >> > 
> >> >> > You really shouldn't be casting much anyway. It's the sort of
> >> 
> >> feature
> >> 
> >> >> > where you're only supposed to use it when you know what you're
> >> 
> >> doing
> >> 
> >> >> > when you use it. And if there's really any possibility that you're dealing with immutable, perhaps you should be casting to const
> >> 
> >> rather
> >> 
> >> >> > than mutable. Personally, I find how C++ created multiple types of
> >> >> 
> >> >> cast
> >> >> 
> >> >> > (include const_cast)_highly_ annoying and generally useless, and I'm_very_ glad that D didn't do anything of the sort.
> >> >> > 
> >> >> > - Jonathan M Davis
> >> >> 
> >> >> "you're only supposed to use it when you know what you're doing when
> >> 
> >> you
> >> 
> >> >> use it"
> >> >> Well, that's kinda the requirement for any feature, isn't it?... :P
> >> >> Well, kinda, I do know what you mean. Yes, one needs to properly
> >> 
> >> learn
> >> 
> >> >> how cast() works, but that doesn't mean it would not be better for
> >> 
> >> the
> >> 
> >> >> feature to be designed in a way that is easier to learn (and just as powerful to use), or actually, it doesn't mean that even if you do
> >> 
> >> know
> >> 
> >> >> how cast() works, that you are not still likely to make a mistake
> >> 
> >> when
> >> 
> >> >> using it.
> >> > 
> >> > Really, you should only be using casts when you need a cast, and you
> >> > should be
> >> > careful when you use them. So, you have to know what you're doing and
> >> > why in
> >> > that particular instance and not use them willy-nilly. In most code,
> >> > casts
> >> > should be quite rare. If you're doing a lot of low-level stuff, then
> >> 
> >> they
> >> 
> >> > might be more common, but in general, they're a sledgehammer that
> >> > shouldn't be
> >> > used except when you really need them.
> >> 
> >> Casting is the recommended way to determine if something is actually a derived type:
> >> 
> >> interface I {}
> >> 
> >> class C : I {}
> >> 
> >> void foo(I i) // change this to const(I) and you have a huge const bug!
> >> {
> >> if(C c = cast(C)i)
> >> {
> >> // optimized branch
> >> }
> >> else
> >> {
> >> // default branch
> >> }
> >> }
> >> 
> >> so it's not so easy to say you should "never" cast. BTW, dcollections uses this quite a bit to support operations between multiple types of collections.
> >> 
> >> It would be nice if at least dynamic cast (with the added rules to
> >> prevent
> >> casting away const/immutable/shared) was a different syntax from cast,
> >> since it's a completely safe usage of casting (as long as you manually
> >> forward modifiers). It's as safe as to! is when used with the right
> >> context.
> > 
> > No. It's not the case that you should _never_ cast, but in most
> > programs, it
> > should still be rare. And yes, casting is the proper way to determine
> > whether
> > an object's type, is derived from a particular class, but it's generally
> > considered bad OO programming to have to ask that sort of question very
> > often.
> > 
> > There are a few places that you're forced to (such as opEquals), but in
> > general, it's a symptom of a bad design. Code should generally be
> > written in
> > such a manner that you either know what it's type is or don't care. You
> > shouldn't generally have to ask it. Obviously, there are exceptions, but
> > they
> > should be exceptions rather than the norm. As such, casts should be
> > fairly
> > rare.
> 
> Any place you have double-dispatch, you need to use it in the dispatch function.
> 
> > Personally, I find the added complexity of C++'s special casts to _far_
> > outweight what benefit they give you. Yes, having something like Java's
> > instanceof instead of having to cast to check whether an object's type is
> > derived from a particular type would arguably be an improvement, but
> > even it
> > shouldn't be used all that often, and I definitely don't think that
> > creating a
> > set of cast operators for different types of cast would be a good idea
> > at all.
> 
> added complexity? How so? There are four types of casts:
> 
> static_cast -> cast without discarding const or volatile
> const_cast -> cast only removing const or volatile
> dynamic_cast -> do a runtime cast to see if an object is actually a
> derived type
> reinterpret_cast -> force everything, and avoid any cast overloads.
> 
> In all of these, only const_cast and reinterpret_cast are unsafe. Each one clearly separates what you wish to do.
> 
> this is not complication, this is expressiveness. Something that D's casting does not have. D's casting conflates multiple things together, and because of the "I trust you" nature that casts have, you can easily make mistakes without complaint from the compiler. The fact that a dynamic cast is inherently unsafe in D is unnecessary. I don't know if it will change, but I can't say it's better than C++. It can be made better than C++, but I think I'd rather have the four cast types than what D has now.

It is definitely added complexity. Now, it allows you to be more explicit and what you want and so it does add expressiveness as well, but with one cast, you know which cast you need to use. With 4, you have to figure out which one to use in a particular instance. Honestly, in discussing the C++ special casts with other programmers in the past, I've gotten the impression that your average C++ programmer does not understand them all, and many of them just use C-style casts all the time. I care about such details in a language and try and understand them, and I confess that I still am not really sure of the exact difference between static_cast and reinterpret_cast and when you're supposed to use one or the other - especially when casting pointers. const_cast and dynamic_cast are easy enough, but having 4 casts instead of 1 where the compiler just does the right thing is definitely added complexity, and it's highly subjective as to whether the added expressivness is worth the complexity. I don't think that it is. You obviously disagree. And for better or worse, D took the route of simplicity rather than expressiveness in this case.

- Jonathan M Davis

On Thu, 02 Jun 2011 19:41:18 -0400, Jonathan M Davis <jmdavisProg@gmx.com> wrote:

> On 2011-06-02 16:22, Steven Schveighoffer wrote:

>> this is not complication, this is expressiveness. Something that D's
>> casting does not have. D's casting conflates multiple things together,
>> and because of the "I trust you" nature that casts have, you can easily
>> make mistakes without complaint from the compiler. The fact that a
>> dynamic cast is inherently unsafe in D is unnecessary. I don't know if it
>> will change, but I can't say it's better than C++. It can be made better
>> than C++, but I think I'd rather have the four cast types than what D has
>> now.
>
> It is definitely added complexity. Now, it allows you to be more explicit and
> what you want and so it does add expressiveness as well, but with one cast,
> you know which cast you need to use.

Think about ref.  Ref has two purposes, passing a value by reference because you are going to use it as a return value, and passing a value by reference because it's cheaper to pass by reference.  But the compiler cannot tell what you mean, so things like rvalues cannot be passed by reference, even if it's for the latter case (because it's cheaper).  In C++, they "fixed" this by allowing const references to bind to rvalues, but the problem there is, you don't know if the person wanted to allow rvalues through.  There is no way to specify "I only want cheaply passed lvalues here!"

What results is, it's difficult to determine the *intentions* of the user given the code he wrote, both for the compiler and the reviewer.  This leads to possible reliance on documentation, and no enforcement.  This is what I'd call false simplicity.  Yes, you didn't need to add another keyword, but is it really simpler?  What happens is, it's simpler to make bugs, and impossible to express what you mean in some cases.

So is giving more expressive meaning to casts adding complexity?  I look at it this way: it results in more thinking on the front end (which cast should I use?) vs. 10x more thinking on the back end (what the hell did the author mean by this cast?  Is it really supposed to strip out const?).  Note also, the front end rules are defined by the C++ standard, whereas the reasoning for using a generic all-in-one cast is only locked inside the author's head.  Hopefully he's documented that for you.

> With 4, you have to figure out which one
> to use in a particular instance. Honestly, in discussing the C++ special casts
> with other programmers in the past, I've gotten the impression that your
> average C++ programmer does not understand them all, and many of them just use
> C-style casts all the time.

Such easy-outs are part of the problem -- if you make the more dangerous path easier than the more expressive and safer path, people give up trying to learn the system.  They might even think they are using clever shortcuts.  Without such an easy out, you will learn the other casts eventually.  I've been a part of teams where C-style casts are not an option, and you just figure it out.

> I care about such details in a language and try
> and understand them, and I confess that I still am not really sure of the
> exact difference between static_cast and reinterpret_cast and when you're
> supposed to use one or the other - especially when casting pointers.

I admit too, I was not correct in remembering how reinterpret_cast works exactly (my C++ skills are suffering from bit-rot) -- it does not allow casting away of const.  However, the rest of my description is accurate.  It basically treats the data as if it was the given type without any implicit or explicit conversions.  For example, if you static_cast an int to a float, it create a float out of an int (i.e 5 turns into 5.0).  However, if you reinterpret_cast an int to a float, it will not change the bit pattern at all, what you get is what a float would be with that bit pattern.

BTW, as far as I can tell, static or reinterpret casting pointers is identical, you should prefer static_cast however because it's safer.

The hilarious thing about reinterpret_cast is, using the casted data is undefined.  All you are allowed to do with it is cast back to the original data.  This may seem useless but gets around some static typing limitations (for example, if you have functions that accept void * data, and you have to override that function, you reinterpret_cast the data to void *, then reinterpret_cast it back on the other end).

I feel D does not need an equivalent reinterpret_cast.  You can achieve the same thing via pointer casting (e.g. *(cast(T*)&u) ).  Note this is entirely possible in D because of the inability to override the & operator.

D already has a clearly delineated const_cast (the subject of this thread).  The problem is, all other cast types are munged together, with const_cast thrown in for good measure.  I think if we stopped debating the value of C++ casts, and simply tried to find a decent syntax for separating out dynamic_cast, we could have all these options without the verbosity.  I'd even go as far to say that dynamic_cast can be a library function -- it's not really a cast.  It can even be made a part of std.conv.to.

-Steve

P.S. I used the information from this SO article to help me write this:  http://stackoverflow.com/questions/332030/when-should-static-cast-dynamic-cast-and-reinterpret-cast-be-used