How to define opCast from native types

[Cherry]

Greetings

> D only lets you define operator overloads in methods, unlike 
> C++ which lets you define overloads as functions. If you want 
> such an operator overload, you will have to define it as a 
> method.

That is all right, since for binary operators there is opBinary 
and opBinrayRight. Correspondingly, I am missing opCastRight.

> However, I suggest that most of the time, writing an opCast 
> overload is usually the wrong solution. It's often better to 
> write functions for type conversions instead, which can easily 
> be defined outside of the struct definition and used with UFCS.
>
> So you can have say this...
>
>     ll.toBoxedInteger
>
> ... instead of this
>
>     cast(Integer) ll
>

In general it seems ".to" conversion is not meant to *cast* away 
information. For example, the following code gives me runtime 
error for trying to convert a long into an int.

void main() {
   import std.conv;
   int ii = 8589934592L.to!int;
}

> However I would suggest that even in this case converting from 
> long to int is error prone, so I would just force users of your 
> struct to cast first from long to int, then wrap the number in 
> your boxed struct.
>
>     Integer(cast(int) ll)
>
> You are much more likely to get warnings from D compilers about 
> the cast in the right places now. However you can go one step 
> further and ask yourself, do you need boxed integers at all? D 
> allows you to create collections of primitive types and so on, 
> so you might not even need a boxed type like Integer, however 
> there may be some use cases.

I just created Integer for the purpose of illustration. I am 
working on a BitVector module. And since BitVector is a template 
(with bitnum parameter), forcing the users to cast into int does 
not work. Consider the case when bitnum is 48 bits.

alias BitVector!48 bitv48;
long ll;
bitv48 foo = cast(bitv48) ll;    // I need cast here


Regards
- Cherry