http://d.puremagic.com/issues/show_bug.cgi?id=3383

           Summary: newVoid
           Product: D
           Version: 2.033
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: patch, performance
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: dsimcha@yahoo.com


--- Comment #0 from David Simcha <dsimcha@yahoo.com> 2009-10-09 10:08:58 PDT ---
D's new keyword for allocating dynamic arrays initializes the data to T.init. This is a perfectly reasonable safe default.  However, there should be an obvious way to optimize this out if one is sure one doesn't need it, as there is for static arrays.  Below is a proposed function, newVoid(), that should go in std.array to allow such a thing.

import core.memory;

/**Gives the block attribute that a block containing type T should have,
 * i.e. scan or NO_SCAN.*/
GC.BlkAttr blockAttribute(T)() {
    if(typeid(T).flags & 1) {
        return cast(GC.BlkAttr) 0;
    } else {
        return GC.BlkAttr.NO_SCAN;
    }
}

unittest {
    assert(blockAttribute!(uint)() == GC.BlkAttr.NO_SCAN);
    assert(blockAttribute!(void*)() == cast(GC.BlkAttr) 0);
}

/**Returns a new array of type T w/o initializing elements.
 *
 * Examples:
 * ---
 * auto foo = newVoid!uint(5);
 * foreach(i; 0..5) {
 *     foo[i] = i;
 * }
 * ---
 */
T[] newVoid(T)(size_t length) {
    T* ptr = cast(T*) GC.malloc(length * T.sizeof, blockAttribute!(T)());
    return ptr[0..length];
}

unittest {
    // Mostly just see if this instantiates.
    auto foo = newVoid!uint(5);
    foreach(i; 0..5) {
        foo[i] = i;
    }

    foreach(i; 0..5) {
        assert(foo[i] == i);
    }
}

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |andrei@metalanguage.com
         AssignedTo|nobody@puremagic.com        |andrei@metalanguage.com


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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|andrei@metalanguage.com     |dsimcha@yahoo.com


--- Comment #1 from Andrei Alexandrescu <andrei@metalanguage.com> 2011-06-05 08:18:18 PDT ---
Reassigning this to David. David, this is a sensible primitive. You may want to make it into a pull request, probably in std.array or even in druntime. A few notes:

1. newVoid is not very indicative, I'd suggest something long and
obvious-looking (as this can be an unsafe function), e.g.
makeUninitializedArray!int(100).

2. Since 0 seems to be an actually used constant of GC.BlkAttr, you may as well give it a name, e.g. NONE.

Thanks!

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #2 from bearophile_hugs@eml.cc 2011-06-05 12:29:45 PDT ---
Nicer and cleaner:

// doesn't initialize foo1
auto foo1 = new uint[5] = void;

// initializes the dynamic array to 5, avoiding a double initialization auto foo2 = new uint[5] = 5;

That syntax is inspired by static array syntax:
uint[5] a1 = void;
uint[5] a2 = 5;

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--- Comment #3 from bearophile_hugs@eml.cc 2011-06-06 03:26:11 PDT ---
One case worth considering:

auto mat = new int[5][5];
foreach (row; mat)
    row[] = 10;

The way used to initialize a matrix to void is useful to initialize all of it to a defined value too.

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David Simcha <dsimcha@yahoo.com> changed:

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


--- Comment #4 from David Simcha <dsimcha@yahoo.com> 2011-08-12 20:43:48 PDT ---
This evolved into std.array.uninitializedArray.

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