Returning a struct by reference
Hi there!
I'm quite new at D and I'm still just playing with it, but there is a thing
that I find currently missing. Sometimes, I'd like to be able to return a
struct by reference and not by value. For example, in the following example:
struct Position {
float x;
float y;
}
class Object {
private Position m_position;
public Position position() {
return m_position;
}
}
I'd like to be able to write things like this: myObject.position.x = 43 to
actually change the position of the object. But right now, since position is
a struct, it is returned by value and not by reference, and then the previous
instruction won't change the position of the object, but it will work on a copy
of the position field.
Here is the solutions that I can see to this problem:
- Returning a pointer to the position: public Position *position() { ... },
but I'd like to keep my code as free from pointers as possible.
- Make Position a class and not a struct. That could be a solution, but
then, when I'll do things like Position pos = object.position; pos.x = 43;,
it will effectively change the position of the object, which I wouldn't like
with this syntax.
Actually, I'd like to be able to do a thing like this:
public ref Position position() {
return m_position;
}
which would be the equivalent form to passing structs by reference in a
parameter.
Is there a way to do this in D?
Regards,
Simon
Re: Returning a struct by reference
Simon TRENY wrote:
Hi there!
I'm quite new at D and I'm still just playing with it, but there is a thing
that I find currently missing. Sometimes, I'd like to be able to return a
struct by reference and not by value. For example, in the following example:
struct Position {
float x;
float y;
}
class Object {
private Position m_position;
public Position position() {
return m_position;
}
}
I'd like to be able to write things like this: myObject.position.x = 43 to actually
change the position of the object. But right now, since position is a struct,
it is returned by value and not by reference, and then the previous instruction won't
change the position of the object, but it will work on a copy of the position field.
Here is the solutions that I can see to this problem:
- Returning a pointer to the position: public Position *position() { ... },
but I'd like to keep my code as free from pointers as possible.
- Make Position a class and not a struct. That could be a solution, but then, when
I'll do things like Position pos = object.position; pos.x = 43;, it will effectively
change the position of the object, which I wouldn't like with this syntax.
Actually, I'd like to be able to do a thing like this:
public ref Position position() {
return m_position;
}
which would be the equivalent form to passing structs by reference in a
parameter.
Is there a way to do this in D?
Yes. Make the variable public.
class Object {
Position position;
}
This code is even simpler than your's above. Incredible, isn't it?
Regards,
Simon
Re: Returning a struct by reference
Simon TRENY wrote: Ok, but then, what if I'd like to make the variable read-only? i.e. preventing the user from writing things like this: myObject.position = pos2; So... you're rejecting a solution on the basis that it prevents you from doing the exact opposite of what you want to do? *boggle* -- Daniel
Re: Returning a struct by reference
grauzone Wrote:
Simon TRENY wrote:
Hi there!
I'm quite new at D and I'm still just playing with it, but there is a thing
that I find currently missing. Sometimes, I'd like to be able to return a
struct by reference and not by value. For example, in the following example:
struct Position {
float x;
float y;
}
class Object {
private Position m_position;
public Position position() {
return m_position;
}
}
I'd like to be able to write things like this: myObject.position.x = 43 to
actually change the position of the object. But right now, since position
is a struct, it is returned by value and not by reference, and then the
previous instruction won't change the position of the object, but it will
work on a copy of the position field.
Here is the solutions that I can see to this problem:
- Returning a pointer to the position: public Position *position() { ...
}, but I'd like to keep my code as free from pointers as possible.
- Make Position a class and not a struct. That could be a solution, but
then, when I'll do things like Position pos = object.position; pos.x =
43;, it will effectively change the position of the object, which I
wouldn't like with this syntax.
Actually, I'd like to be able to do a thing like this:
public ref Position position() {
return m_position;
}
which would be the equivalent form to passing structs by reference in a
parameter.
Is there a way to do this in D?
Yes. Make the variable public.
class Object {
Position position;
}
This code is even simpler than your's above. Incredible, isn't it?
Ok, but then, what if I'd like to make the variable read-only? i.e.
preventing the user from writing things like this:
myObject.position = pos2;
Regards,
Simon
Re: Returning a struct by reference
Simon TRENY wrote:
grauzone Wrote:
Simon TRENY wrote:
Hi there!
I'm quite new at D and I'm still just playing with it, but there is a thing
that I find currently missing. Sometimes, I'd like to be able to return a
struct by reference and not by value. For example, in the following example:
struct Position {
float x;
float y;
}
class Object {
private Position m_position;
public Position position() {
return m_position;
}
}
I'd like to be able to write things like this: myObject.position.x = 43 to actually
change the position of the object. But right now, since position is a struct,
it is returned by value and not by reference, and then the previous instruction won't
change the position of the object, but it will work on a copy of the position field.
Here is the solutions that I can see to this problem:
- Returning a pointer to the position: public Position *position() { ... },
but I'd like to keep my code as free from pointers as possible.
- Make Position a class and not a struct. That could be a solution, but then, when
I'll do things like Position pos = object.position; pos.x = 43;, it will effectively
change the position of the object, which I wouldn't like with this syntax.
Actually, I'd like to be able to do a thing like this:
public ref Position position() {
return m_position;
}
which would be the equivalent form to passing structs by reference in a
parameter.
Is there a way to do this in D?
Yes. Make the variable public.
class Object {
Position position;
}
This code is even simpler than your's above. Incredible, isn't it?
Ok, but then, what if I'd like to make the variable read-only? i.e.
preventing the user from writing things like this:
myObject.position = pos2;
Then you write a getter that simply returns the field by value.
The D compiler will (hopefully) inline the getter function, so there
shouldn't be a disadvantage in performance.
Note: I think D2.0 wants to introduce ref-returns at some point in the
future.
Regards,
Simon
Re: Returning a struct by reference
Daniel Keep Wrote:
Simon TRENY wrote:
Ok, but then, what if I'd like to make the variable read-only? i.e.
preventing the user from writing things like this:
myObject.position = pos2;
So... you're rejecting a solution on the basis that it prevents you from
doing the exact opposite of what you want to do?
*boggle*
-- Daniel
Here is a complete example of what I'd like to achieve:
struct Position {
private float m_x;
private float m_y;
public float x() {
return m_x;
}
public void x(float x) {
m_x = x;
EmitSignal(changed);
}
public float y() {
return m_y;
}
public void y(float y) {
m_y = y;
EmitSignal(changed);
}
}
class Object {
private Position m_position;
public this() {
m_position.CallOnSignal(changed, onPositionChanged);
}
//This syntax is not working
public ref Position position() {
return m_position;
}
public void onPositionChanged() {
writeln(Position Changed!!);
}
}
With this fictional code, I could write things like:
object.position.x = 14; and the object will be aware that its position has
changed.
Making the position-variable public will lead the user to be able to do
things like this:
object.position = pos2; and then, the object won't be aware that its position
has changed. And this is a problem for me.
I hope it's clearer now
Re: Returning a struct by reference
grauzone Wrote:
Simon TRENY wrote:
grauzone Wrote:
Simon TRENY wrote:
Hi there!
I'm quite new at D and I'm still just playing with it, but there is a
thing that I find currently missing. Sometimes, I'd like to be able to
return a struct by reference and not by value. For example, in the
following example:
struct Position {
float x;
float y;
}
class Object {
private Position m_position;
public Position position() {
return m_position;
}
}
I'd like to be able to write things like this: myObject.position.x = 43
to actually change the position of the object. But right now, since
position is a struct, it is returned by value and not by reference, and
then the previous instruction won't change the position of the object,
but it will work on a copy of the position field.
Here is the solutions that I can see to this problem:
- Returning a pointer to the position: public Position *position() { ...
}, but I'd like to keep my code as free from pointers as possible.
- Make Position a class and not a struct. That could be a solution,
but then, when I'll do things like Position pos = object.position; pos.x
= 43;, it will effectively change the position of the object, which I
wouldn't like with this syntax.
Actually, I'd like to be able to do a thing like this:
public ref Position position() {
return m_position;
}
which would be the equivalent form to passing structs by reference in a
parameter.
Is there a way to do this in D?
Yes. Make the variable public.
class Object {
Position position;
}
This code is even simpler than your's above. Incredible, isn't it?
Ok, but then, what if I'd like to make the variable read-only? i.e.
preventing the user from writing things like this:
myObject.position = pos2;
Then you write a getter that simply returns the field by value.
The D compiler will (hopefully) inline the getter function, so there
shouldn't be a disadvantage in performance.
If I add a getter-property that returns the field by value, the following
instruction object.position.x = 12; won't modify the position of the object,
but will only modify the returned copy of the position, right?
That's actually why I'd like to have a getter that returns the field by
reference and not by value.
Note: I think D2.0 wants to introduce ref-returns at some point in the
future.
Regards,
Simon
Re: Returning a struct by reference
Sat, 21 Mar 2009 09:55:13 -0400, Simon TRENY wrote:
//This syntax is not working
public ref Position position() {
return m_position;
}
D2 supports this.
D1 won't, ever. I think.
Re: Returning a struct by reference
Simon TRENY wrote:
Daniel Keep Wrote:
Simon TRENY wrote:
Ok, but then, what if I'd like to make the variable read-only? i.e.
preventing the user from writing things like this:
myObject.position = pos2;
So... you're rejecting a solution on the basis that it prevents you from
doing the exact opposite of what you want to do?
*boggle*
-- Daniel
Here is a complete example of what I'd like to achieve:
struct Position {
private float m_x;
private float m_y;
public float x() {
return m_x;
}
public void x(float x) {
m_x = x;
EmitSignal(changed);
}
public float y() {
return m_y;
}
public void y(float y) {
m_y = y;
EmitSignal(changed);
}
}
class Object {
private Position m_position;
public this() {
m_position.CallOnSignal(changed, onPositionChanged);
}
//This syntax is not working
public ref Position position() {
return m_position;
}
public void onPositionChanged() {
writeln(Position Changed!!);
}
}
With this fictional code, I could write things like:
object.position.x = 14; and the object will be aware that its position has
changed.
Making the position-variable public will lead the user to be able to do
things like this:
object.position = pos2; and then, the object won't be aware that its position
has changed. And this is a problem for me.
But if position is returned as ref, this still could happen. The
returned value is still assignable, and because it's a ref, overwriting
it is like overwriting m_position directly. As far as I see, your
position()-getter just emulates a public field. Including overwriting by
assignment.
Now D2.0 has const. If position() would return a const object, this kind
of unwanted overwriting couldn't happen. This improves correctness,
because uncatched changes or changes to temporaries can't happen. But
then again, the setters in the Position struct wouldn't work, because
the struct is const (as far as I understand the const system). This
means returning the field as a const ref wouldn't help.
Also, how is this EmitSignal() working? What happens if you write:
Position p = object.position;
p.x = 14; //EmitSignal() calls what?
p is simply a bit-copy of the struct returned by the getter, and
EmitSignal() has no way to check if it's still supposed to notify class
Object. (All solutions I can come up with sound really hairy and hackish.)
Maybe you're better off with Position as a class instead of a struct.
Even when you use D2.0's const/opAssign/copy-ctor/post-blit/ref-return
features.
Looking forward to the replies pointing out that D2.0 actually allows to
implement exactly what you want, and how this is done.
I hope it's clearer now
Re: Returning a struct by reference
Hello Simon,
If I add a getter-property that returns the field by value, the
following instruction object.position.x = 12; won't modify the
position of the object, but will only modify the returned copy of the
position, right?
That's actually why I'd like to have a getter that returns the field
by reference and not by value.
That is correct. Reference returns are on the todo list. For now this Hack
should work.
struct S { float x; float y; }
class C
{
S s
class C_S { void x(float v){ s.x=v; } void y(float v){ s.y=v; } }
C_S pos() { return new C_S(); }
}
