On Wednesday, 7 November 2012 at 02:34:25 UTC, martin wrote:
On Wednesday, 7 November 2012 at 02:06:09 UTC, Rob T wrote:
What about the case where we want to pass a source argument
either by reference or as a copy depending on the l/r value
situation?
eg
void f( ref a );
void f( a );
--rt
I don't get what you mean - that's why the 2 overloads are for
(you forgot the const/in keyword - beware! :))
void f( in ref T a );
void f( in T a );
rvalue binds to the latter overload (argument not copied, but
moved directly).
lvalue binds to the first overload (reference).
Works with v2.060.
Sorry, my explanation was very poor. I'll try again.
I'm trying to describe the case where you want to modify the ref
argument so that the source is also changed. For example, let's
say you want to move a resource instead of coping it. Moving
means the resource is transfered from one object to another, and
the original is reset without destoying the resource. That way
only one object can have the same resource. So for moving a
resource, you cannot use "in ref" or "const ref", you need to
just use "ref".
void f( ref T a ){
// take resource away from a
this.resource = a.resource;
a.resetResource();
// this now owns the resource
}
ref T works fine, but if you wish to use f( ref T a ) on a temp
value returned from another function call, you'll need to
overload f() to pass by value, which means creating a duplicate.
void f( ref T a ){
// take resource away from a
this.resource = a.resource;
a.resetResource();
// this now owns the resource
}
Example:
T g(){
T temp;
// create resource which is stored inside T
temp.createResource();
// move temp contents to return value
return move(temp);
}
f( g() ); // won't compile with ref
It's annoying to have to create overloaded duplicates for a
situation like moving, esp if the duplicate is a lot of code. In
C++ the move problem was solved using && move semantics, and this
is what I'm trying to emulate in D but I feel like I'm fighting
with the language.
It is possible that D has a specific way of solving this problem
that I'm not aware of yet, so correct me if I'm asking for
something that is simply not needed.
I tried f( move(g()) ) but that fails to work. My best guess is
that D does a hidden move of the temp instead of a copy to value.
I can't say for sure because the documentation is not clear and
is missing important details like this. I also cannot rely on
clever compiler optimizations that may or may not be implemented
as a guarantee.
--rt
