[Issue 8006] New: Implement proper in-place-modification for properties

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8006

           Summary: Implement proper in-place-modification for properties
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2012-04-29 23:26:40 PDT ---
Currently properties are only usable for reading and writing values, but they
can't really be used for in-place modification:

// fake int type, just to avoid rvalue errors in this demo
struct Bar { int x; alias x this; }

struct Foo
{
    Bar _val;
    @property Bar val() { return _val; }
    @property void val(Bar nval) { _val = nval; }
}

void main()
{
    Foo foo;
    foo.val += 5;  // modifies *temporary*, then discards it
    foo.val++;  // ditto
}

The only way to work around this is to make the getter property return by ref,
but this completely circumvents the setter property, e.g.:

struct Bar { int x; alias x this; }
struct Foo
{
    Bar _val;
    @property ref Bar val() { return _val; }
    @property void val(Bar nval) { _val = nval; }  // never called
}

void main()
{
    Foo foo;
    foo.val += 5;
    assert(foo.val == 5);  // updated, but setter circumvented
    foo.val++;
    assert(foo.val == 6);  // ditto
}

C# apparently implements in-place modification by translating calls such as
this:
foo.val += 5;
foo.val++;

into this:
foo.val = foo.val + 5;
foo.val = foo.val + 1;

DIP4 also mentioned this feature
(http://www.prowiki.org/wiki4d/wiki.cgi?LanguageDevel/DIPs/DIP4), but was
superseeded by DIP6 which was approved. I think we ought to implement this to
make properties more usable and less error-prone to work with.

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