On Tuesday, 26 June 2012 at 15:40:48 UTC, Jonathan M Davis wrote:
On Tuesday, June 26, 2012 14:44:45 monarch_dodra wrote:
Depending on your type, taking const ref Widget would be a big
problem,
because you wouldn't be able to assign a const member
variable's value of rhs
to a non-const member variable of this (e.g. if it's a class)
without making a
deep copy. Or it could be just fine. It all depends on your
type and what
you're trying to do. Regardless, the solution is almost
certainly to have
multiple overloads. Each of the overloads does this
// Works with any Widget if Widget is a value type and any
non-const Widget
// if it's a reference type.
ref Widget opAssign(Widget rhs) {}
// Works with any Widget as long as you can copy all of the
member variables
// when they're const.
ref Widget opAssign(const Widget rhs) {}
// Works with non-const Widgets which are lvalues only.
ref Widget opAssign(ref Widget rhs) {}
// Works with any Widgets which are lvalues as long as you can
copy all of the
// member variables when they're const.
ref Widget opAssign(ref Widget rhs) {}
In most cases, I would expect you to have these two overloads
ref Widget opAssign(const Widget rhs) {}
ref Widget opAssign(const ref Widget rhs) {}
It then works with const (as long as you're not dealing with a
type which
can't really be copied when it's const) and both rvalues and
lvalues. The
rvalue version must be const to ensure that constness does not
affect which
overload gets called (just l/rvalue-ness). This is particularly
critical if
the rvalue version simply calls the lvalue version, because if
it's not const,
you'd get infinite recursion.
- Jonathan M Davis
Thanks for the in-depth explanation! It is still not very clear
to me, mostly because I don't understand "you wouldn't be able to
assign a const member variable's value of rhs to a non-const
member variable of this". This works fine in C++. Must be because
I've never used a language with reference semantics before.
Either that, or I'm miss-understanding the "ref" keyword. I
thought "const ref" meant pass by const reference, but it would
appear it means a reference to a const object... or something
like that. I'll need to read the chapter on const too.
Either way, I guess I'll have to become more familiar with the
language to fully appreciate this.
