Christopher Jefferson wrote:
On 3 Sep 2009, at 03:43, Andrei Alexandrescu wrote:
Hello,
I'm defining an "optional" type for D's standard library modeled similarly to Boost.optional. An interesting question came up - should optional<optional<T>> fold itself into optional<T>, or is "double optional" an interesting concept of its own?
I thought I'd ask here because by now there's a significant body of experience with optional<T>. I perused the online documentation and the forum and couldn't find information about that specific detail.
Only a related note, I use a version of optional which takes the "m_initalized", effectively the boolean that tells you if a value is present, as a template parameter.
This is because for many types, it is possible to know there is some value in the object itself which cannot take certain values, so can be overloaded.
As an example, given a vector<T> implemented as:
struct vector<T> { T* begin; T* end; size_t length };
Then I know that (on my system) begin will never take (void*)(1), (it might be NULL), so I can reuse that to implement optional<T> without any extra space usage.
The main problem with this, at least in C++, this is basically impossible to implement generically and legally, as you are effectively poking into an objects memory without knowing what it is, so I haven't tried to carefully formalise it.
Chris
Yah, good point. The solution I had in mind for that was to also define a two-parameter Optional: Optional<T, T nullValue>. Then if you want to use it with e.g. getchar(), you'd use Optional<int, EOF>. Unfortunately, in C++ this is not as general as it could because T is limited to a few types (D does not have this particular restriction). Something I think could work nicely for C++ would be to parameterize on an address, e.g. Optional<T, const T* nullValuePtr>. Continuing on the getchar example, this could work: const int EOF_global = EOF; typedef Optional<int, &EOF_global> GetcharResult; In your case, you'd have to define a global vector object with the pointers forged appropriately. Andrei