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digitalmars.D - Returning a struct by reference

reply Simon TRENY <simon.treny free.fr> writes:
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
Mar 21 2009
parent reply grauzone <none example.net> writes:
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
Mar 21 2009
parent reply Simon TRENY <simon.treny free.fr> writes:
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
Mar 21 2009
next sibling parent Daniel Keep <daniel.keep.lists gmail.com> writes:
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
Mar 21 2009
prev sibling parent reply grauzone <none example.net> writes:
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?




 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
Mar 21 2009
parent reply Simon TRENY <simon.treny free.fr> writes:
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?




 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
Mar 21 2009
parent BCS <none anon.com> writes:
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(); }
}
Mar 21 2009