[Issue 1596] New: op*Assign should return void

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1596

           Summary: op*Assign should return void
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: andrei at metalanguage.com


D's assignment operators opAddAssign, opMulAssign etc. have taken part of C++'s
behavior in the area. In order to behave similarly to a primitive type, a
user-defined operator typically returns *this as an rvalue (in C++ they'd
usually return an lvalue).

There are a few problems with this approach:

1. Extra busywork done for no good reason. The code:

a += b;

translates to:

typeof(a) __unused = void;
a.opAddAssign(b, __unused);

So there is one extra memcpy performed for no reason (the copy is done on the
caller site, which has no idea whether the result of the operation will be used
or not). This problem will be exacerbated when copy construction semantics will
be introduced.

2. Extra work for the programmer, and relying on convention instead of language
rules.

Experience with C++ shows that user-defined implementations of assignment
operators always return *this, and that there is absolutely no reason to return
anything else. 

Fix:

The rule belongs to the language. Require all op*Assign to return void. If a
value is required on the caller side, pass the left-hand side of the operation.


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