• d-bugmail puremagic.com Oct 19 2007
• Don Clugston <dac nospam.com.au> Oct 23 2007

d-bugmail puremagic.com writes:

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 digitalmars.com
        ReportedBy: andrei 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.


-- 

Don Clugston <dac nospam.com.au> writes:

d-bugmail 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 digitalmars.com
         ReportedBy: andrei 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.