Re: [boost] [optional] Safe optional

Subject: Re: [boost] [optional] Safe optional
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-11-23 18:14:04

On 21/11/2014 11:42, Olaf van der Spek wrote:
> What do other languages do? Do they have something like optional? How
> are comparisons defined?

In .NET, Nullable<T>:

  - has implicit T-to-Nullable<T> conversion & construction

  - has explicit Nullable<T>-to-T conversion (which throws an exception 
if it has no value)

  - can either access "HasValue" and "Value" properties directly or use 
a "value_or" style method.

  - supports equality testing (two nulls are equal); due to the way that 
default language conversions work this applies to both T and Nullable<T>.

  - supports "getting a suitable hash value" as an operation, similar to 
std::hash.

  - does NOT support comparisons other than equality directly, although 
there is an explicit comparison method Nullable.Compare<T> that can be 
called to do so; it operates only on two Nullable<T>s but implicit 
conversion will allow a T to be passed in unadorned.