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digitalmars.D.learn - Nested Structs

reply d coder <dlang.coder gmail.com> writes:
Greetings All

I have a situation where I have a struct nested inside a class. I
would like to make the enclosing class' members visible inside the
nested struct's constructor. I see that such preposition is feasible
for nested classes, but not for nested structs. Why so?

Are there some alternative constructs which could give me this
behavior for nested constructs?

Regards
- Cherry
Dec 27 2010
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 27 Dec 2010 08:54:37 -0500, d coder <dlang.coder gmail.com> wrote:


 Greetings All

 I have a situation where I have a struct nested inside a class. I
 would like to make the enclosing class' members visible inside the
 nested struct's constructor. I see that such preposition is feasible
 for nested classes, but not for nested structs. Why so?


A struct nested in a class does not have a hidden "outer" pointer as a  
nested class does.  It's because a struct is generally more bare-bones  
than a class (which has loads of hidden pieces: vtable, interfaces,  
classinfo, etc.).  Also, instantiating such a struct does not tie it to a  
class instance.


 Are there some alternative constructs which could give me this
 behavior for nested constructs?


You need to implement this behavior on your own:

class C
{
   struct S
   {
     private C _outer;
     this(C c) { this._outer = c; _outer.x = 5;} // access enclosing class  
instance via _outer.
   }

   int x;
}

-Steve
Dec 27 2010
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Steven Schveighoffer:


 A struct nested in a class does not have a hidden "outer" pointer as a  
 nested class does.


But I think this will eventually change, once this part is implemented:
http://www.digitalmars.com/d/2.0/struct.html


Nested Structs: A nested struct is a struct that is declared inside the scope
of a function or a templated struct that has aliases to local functions as a
template argument. Nested structs have member functions. It has access to the
context of its enclosing scope (via an added hidden field).<


Bye,
bearophile
Dec 27 2010
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 27 Dec 2010 12:50:47 -0500, bearophile <bearophileHUGS lycos.com>  
wrote:


 Steven Schveighoffer:


 A struct nested in a class does not have a hidden "outer" pointer as a
 nested class does.


 But I think this will eventually change, once this part is implemented:
 http://www.digitalmars.com/d/2.0/struct.html


 Nested Structs: A nested struct is a struct that is declared inside the  
 scope of a function or a templated struct that has aliases to local  
 functions as a template argument. Nested structs have member functions.  
 It has access to the context of its enclosing scope (via an added  
 hidden field).<




That is implemented, but notice the conditions that form a nested struct  
do not include "inside a class" or "inside a struct" (that "templated  
struct that has aliases to local functions as a template argument"  
condition is pretty specific, I'm talking about just putting it in any old  
struct).

This is decidedly different from a nested class:

http://www.digitalmars.com/d/2.0/class.html#nested

"A nested class is a class that is declared inside the scope of a function  
or another class. A nested class has access to the variables and other  
symbols of the classes and functions it is nested inside"

This is what the OP was desiring.

-Steve
Dec 27 2010
prev sibling parent d coder <dlang.coder gmail.com> writes:
 A struct nested in a class does not have a hidden "outer" pointer as a
 nested class does. =A0It's because a struct is generally more bare-bones =


than

 a class (which has loads of hidden pieces: vtable, interfaces, classinfo,
 etc.). =A0Also, instantiating such a struct does not tie it to a class
 instance.


Thanks Steve

As far as I am concerned, this is a very limiting behavior wrt structs
nested inside classes. I have also observed that if you define a
constant (using enum for example) inside the enclosing class, the
constant remains visible in the nested struct too. So I believe the
language is making the hidden "outer" scope visible to the nested
struct (or maybe there is some other mechanism for enums that I am not
aware of).


 You need to implement this behavior on your own:


Actually I am trying to define a DSEL and the end users are not
typically programmers. I am in a situation where the end-user decides
the unit in an enclosing class (unit of length as an inch for example)
and later whenever he instantiates a line or a shape, the particular
unit is automatically considered at the time of instantiation. For my
application, I need value semantics and therefor using nested classes
in place of structs can not be considered.

Making the end-users pass the unit or its context (in form of this
pointer) everytime they instantiate a shape would be atrocious. It
would defeat the very purpose of letting the end-user define a unit.

I know I am not on the right mailing group, but I want to ask for this
particular language enhancement. I believe this would make the
behavior of the language more straight wrt nested structs inside
classes (are not these beasts expected to serve like nested classes or
even structs nested inside function scopes?). I hope my requirement is
generic enough and fits the bill for an enhancement request.

Regards
- Cherry
Dec 27 2010