🚀 go-pugleaf

In article <1992Oct31.025229.6520@cs.brown.edu> sdm@cs.brown.edu (Scott Meyers) writes:
|Is the return type of operator= an lvalue?  

No.

|In particular, if a, b, and c are of built-in type, is this legal?
|
|    (a = b) = c;

No, it is an error.  If (a = b) were an lvalue the same
as "a" then "a" would be being accessed twice for modification in one
expression, which in turn is an unconstrained error.  

|I couldn't find an answer in the ARM, 

My answers are based on the ANSI-C spec, under the optimistic assumption that 
some day the ANSI/ISO-C++ spec will contain equivalently detailed language about
what kinds of errors can occur in programs, and what actions compilers
can or cannot perform as a result of those errors.

|My goal is to determine the correct type for user-defined operator=.  If
|the return type is to be an lvalue, then for a class C, the return type
|should be C&.  However, if the return value is not to be an lvalue, then
|op= should return a const C&.  Two years ago I thought const C& was the
|right type, but since then I've become convinced that C& is better.  Now
|doubt has surfaced in my mind again, and I'd like to put the matter to
|rest.

The "right" thing to do is to return a value, not a reference.  Given
the infrequent use of the implied value of an assignment operation, 
many C++ programmers are lothe to return by value.  In which case
returning a const reference might be an acceptable hack.  You might return
void instead in cases where programmers shouldn't reasonably expect
to use a side-effect value of assignment.  Don't return a non-const reference.
This would further confuse users about the illegality of modifying a
variable multiple times in one expression.