On Wednesday, 7 November 2012 at 06:13:25 UTC, Rob T wrote:
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.
Duplicating the function, yes, but not duplicating the rvalue
since it is moved in D. Isn't this analog to the C++ solution?
C++:
void f(T& a) { // for lvalues
this->resource = a.resource;
a.resetResource();
}
void f(T&& a) { // for rvalues (moved)
this->resource = a.resource;
a.resetResource();
}
D:
void f(ref T a) { // for lvalues
this.resource = a.resource;
a.resetResource();
}
void f(T a) { // rvalue argument is not copied, but moved
this.resource = a.resource;
a.resetResource();
}
T g() {
T temp;
temp.createResource();
return temp; // D already moves temp!
// 'Named Return Value Optimization'
}
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.
You could implement a copy constructor 'this(this)' in your
struct T and see when it is invoked to check when an instance is
actually copied. I'd expect that invoking 'f(g());' with above
implementation doesn't copy anything.
