www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - strettosis tutanota.com

reply Stretto <Strettosis Gmail.com> writes:
I have some class like

class bar { }
class foo : bar
{
     bar[] stuff;
}

and have another class

class dong : bar
{
    int x;
}


Now sometimes stuff will contain dong's, but I cannot access its 
members it without a cast.

fooo.stuff[0].x // invalid because bar doesn't contain x;

Hence,

((cast(dong)foo.stuff[0]).x is the normal way with a possible 
type check.

But in my case I will never mix different types in stuff and will 
always use it properly or do type checking in the cases I might 
mix.

Rather than add a dong[] dongs; to foo, which increases the size 
of foo and wastes memory just to prevent the cast, I'm curious if 
there is any other way to solve this problem?

I simply want to do foo.stuff[0].x and have foo.stuff[0] be 
treated as an image.

Is an opDispatch and/or opIndex required or is there some alias 
trick that can be used?

I'd rather access like foo.dongs[0].x without defining a dong 
array directly in foo, but simply alias to stuff with an implicit 
cast to dong.
Jun 09 2016
next sibling parent Stretto <Strettosis Gmail.com> writes:
Ultimately what I want to do is access a member

foo.Dongs[i];

Where Dongs is essentially a "view" in to the Bars array and only 
accesses types of type Dong.

It seems one can't do both an override on a name("Dongs") and an 
index on the overridden name(`[i]`)?

It is not appropriate to use foo.Dongs(i).

A clear example:

https://dpaste.dzfl.pl/7ea52a0f21ce
Jun 09 2016
prev sibling parent reply Mike Parker <aldacron gmail.com> writes:
On Thursday, 9 June 2016 at 22:19:33 UTC, Stretto wrote:
 I have some class like

 class bar { }
 class foo : bar
 {
     bar[] stuff;
 }

 and have another class

 class dong : bar
 {
    int x;
 }


 Now sometimes stuff will contain dong's, but I cannot access 
 its members it without a cast.

 fooo.stuff[0].x // invalid because bar doesn't contain x;

 Hence,

 ((cast(dong)foo.stuff[0]).x is the normal way with a possible 
 type check.

 But in my case I will never mix different types in stuff and 
 will always use it properly or do type checking in the cases I 
 might mix.
That's just the nature of working with class hierarchies. A Derived is always a Base, but a Base might not be a Derived. If your Bar array in Foo will always hold only one type of Bar, then you can parameterize Foo with a type: ########################### class Bar { } // Only accept types that are implicitly convertible to Bar class Foo(T : Bar) : Bar { T[] stuff; } class Dong : Bar { int x; this(int x) { this.x = x; } } void main() { import std.stdio; auto foo = new Foo!Dong(); foo.stuff ~= new Dong(10); writeln(foo.stuff[0].x); } ########################### Another option is to use a parameterized getter, which is somewhat cleaner than a cast. ########################### class Foo : Bar { Bar[] stuff; T get(T : Bar)(size_t index) { return cast(T)stuff[index]; } } void main() { import std.stdio; auto foo = new Foo(); foo.stuff ~= new Dong(10); writeln(foo.get!Dong(0).x); } ###########################
Jun 09 2016
parent ArturG <var.spool.mail700 gmail.com> writes:
you could also use a simple wrapped cast

Ret castTo(Ret, T)(T t) if(is(T == class))
{
     return cast(Ret) t;
}

then do

foo.stuff[0].castTo!Dong.x.writeln;

and if you want to guard the access you could try

foo.stuff[0].castTo!Dong.cc!((d){d.x = 5;});

cc is an alias for checkCall which is a template you can find here
http://forum.dlang.org/thread/ltalqpmpscdoziserqqx forum.dlang.org,
it treats Type.init as false and ignores the call to fun.

but its not restricted to nullables only e.g.

float someF;
iota(0, someF).writeln; // normally would throw an AssertError

someF.cc!(f => iota(0, f)).writeln; // returns an empty range 
without calling iota, as float.nan is treated as false.
same as
0.0.cc!(f => iota(0, f)).writeln;
Jun 10 2016