How to define opCast from native types

[w0rp]

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. 
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

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.