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

[Don Clugston]

d-bugmail at puremagic.com wrote:
> 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.

I think this is right. But it's worth mentioning that this would break some of 
my code, since I'm using the return type of opXXAssign() operations in 
expression templates. However, since D expression templates don't work for 
comparison operators (because opCmp() returns an int), changing this is likely 
to force us to a better expression template solution.