const ref in opAssign

[monarch_dodra]

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.