http://d.puremagic.com/issues/show_bug.cgi?id=9265

           Summary: Nullable fixed-sized array wrapper
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2013-01-03 05:25:28 PST ---
When I use fixed-sized arrays a lot, sometimes I need a nullable version of them (fixed-sized arrays are allocated in-place, so they often avoid heap allocations, avoiding the creation of some garbage, reducing the GC pressure and generally speeding up the code). A "ref" can't be used because it can't be null.

So I think the normal way to use them in D is this, but the "(*arr)[1]" syntax is bug-prone and not elegant:


alias TA = immutable(int[5]);
bool foo(TA* arr) {
    if (arr)
        return (*arr)[1] == 20;
    else
        return false;
}
void main() {
    TA a;
    foo(&a);
}



std.typecons.Nullable and std.typecons.NullableRef contain enforce() that
throws and kills inlining.


This simple struct inspired by NullableRef is light, avoids the problem of NullableRef and allows for a natural syntax for nullable fixed sized arrays:



// Nullable fixed-sized array --------
struct Ptr(T) {
    private T* ptr;

    this(T* ptr_) pure nothrow {
        this.ptr = ptr_;
    }

    bool opCast(T)() const pure nothrow if (is(T == bool)) {
        return ptr !is null;
    }

    @property ref inout(T) get()() inout pure nothrow
    in {
        assert(ptr);
    } body {
        return *ptr;
    }

    alias get this;
}

Ptr!T ptr(T)(ref T x) {
    return typeof(return)(&x);
}

// Example usage ----------

alias TA = immutable(int[5]);

bool foo(Ptr!TA arr=Ptr!TA.init) nothrow {
    if (arr)
        return arr[1] == 20;
    else
        return false;
}

bool foo(typeof(null) _) nothrow {
    return false;
}

void main() {
    assert(!foo());
    assert(!foo(null));
    TA items = [10, 20, 30, 40, 50];
    assert(foo(ptr(items)));
}
//---------------------------------



Unfortunately my benchmarks show that with the current DMD 2.061 the code that uses such wrapped array is not as efficient as the first program that uses the pointer to the fixed sized array.

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monarchdodra@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra@gmail.com


--- Comment #1 from monarchdodra@gmail.com 2013-02-04 22:57:48 PST ---
(In reply to comment #0)
> std.typecons.Nullable and std.typecons.NullableRef contain enforce() that
> throws and kills inlining.

FYI: I submitted a pull that changes those enforces to logic errors (asserts):

https://github.com/D-Programming-Language/phobos/pull/1103

This should allow the code to inline.

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--- Comment #2 from github-bugzilla@puremagic.com 2013-03-02 10:06:08 PST ---
Commits pushed to master at https://github.com/D-Programming-Language/phobos

https://github.com/D-Programming-Language/phobos/commit/e2831a5f2a7f3a8b0df4e475ff4f79fea2ff5c0f
Fixes Issue 9265 - assertNotThrown should emit msg from thrown exception if
available.

https://github.com/D-Programming-Language/phobos/commit/ba4095de5f2c6c62dbec45b71ec3e4ba3d802f90 Merge pull request #1185 from AndrejMitrovic/Fix9625

Issue 9265 - assertNotThrown should emit msg from thrown exception if available.

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--- Comment #3 from bearophile_hugs@eml.cc 2013-03-02 12:48:12 PST ---
(In reply to comment #2)
> Commits pushed to master at https://github.com/D-Programming-Language/phobos
> 
> https://github.com/D-Programming-Language/phobos/commit/e2831a5f2a7f3a8b0df4e475ff4f79fea2ff5c0f
> Fixes Issue 9265 - assertNotThrown should emit msg from thrown exception if
> available.
> 
> https://github.com/D-Programming-Language/phobos/commit/ba4095de5f2c6c62dbec45b71ec3e4ba3d802f90 Merge pull request #1185 from AndrejMitrovic/Fix9625
> 
> Issue 9265 - assertNotThrown should emit msg from thrown exception if available.

Is this related?

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich@gmail.com


--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-03-02 13:39:58 PST ---
(In reply to comment #3)
> (In reply to comment #2)
> > Commits pushed to master at https://github.com/D-Programming-Language/phobos
> > 
> > https://github.com/D-Programming-Language/phobos/commit/e2831a5f2a7f3a8b0df4e475ff4f79fea2ff5c0f
> > Fixes Issue 9265 - assertNotThrown should emit msg from thrown exception if
> > available.
> > 
> > https://github.com/D-Programming-Language/phobos/commit/ba4095de5f2c6c62dbec45b71ec3e4ba3d802f90 Merge pull request #1185 from AndrejMitrovic/Fix9625
> > 
> > Issue 9265 - assertNotThrown should emit msg from thrown exception if available.
> 
> Is this related?

No it was a typo in the commit message, sorry.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #5 from bearophile_hugs@eml.cc 2013-03-02 13:42:02 PST ---
Using the updated NullableRef in my code I've seen that it is good enough for the purposes explained here, so I close down this issue.

I'd like a more handy way to create a nullable, like:

Ptr!T ptr(T)(ref T x) {
    return typeof(return)(&x);
}


So I've created the Issue 9637

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