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digitalmars.D.learn - Problem with "delete" and what about "friend" classes & functions?

reply John Kiro <johnkirollos yahoo.com> writes:
Hello

I have a problem with this statement:

  delete pDoc.getFile();

It generates this error:

"Error: ((cast(CDoc)(pDoc)).getFile)() is not an lvalue"

The getFile() is a member of CDoc (pDoc is an object of a CDoc
derived class), and is defined like this:

  CFile getFile()
  {
	return m_pFile;
  }

So any idea about the reason of this error?
The code compiles successfully if I replace the delete statement with:

  CFile tmpFile=pDoc.getFile();
  delete tmpFile;

Another point, I guess D doesn't support "friend" classes or
functions.. wouldn't they be useful sometimes?

Thanks
John
Jan 05 2007
next sibling parent Kirk McDonald <kirklin.mcdonald gmail.com> writes:
John Kiro wrote:

 Hello
 
 I have a problem with this statement:
 
   delete pDoc.getFile();
 
 It generates this error:
 
 "Error: ((cast(CDoc)(pDoc)).getFile)() is not an lvalue"
 
 The getFile() is a member of CDoc (pDoc is an object of a CDoc
 derived class), and is defined like this:
 
   CFile getFile()
   {
 	return m_pFile;
   }
 
 So any idea about the reason of this error?
 The code compiles successfully if I replace the delete statement with:
 
   CFile tmpFile=pDoc.getFile();
   delete tmpFile;
 


'delete' also sets the reference to null. Therefore, it must operate on 
an lvalue.


 Another point, I guess D doesn't support "friend" classes or
 functions.. wouldn't they be useful sometimes?
 


Everything in the same module is a 'friend'.

-- 
Kirk McDonald
Pyd: Wrapping Python with D
http://pyd.dsource.org
Jan 05 2007
prev sibling next sibling parent Sean Kelly <sean f4.ca> writes:
John Kiro wrote:

 Hello
 
 I have a problem with this statement:
 
   delete pDoc.getFile();
 
 It generates this error:
 
 "Error: ((cast(CDoc)(pDoc)).getFile)() is not an lvalue"
 
 The getFile() is a member of CDoc (pDoc is an object of a CDoc
 derived class), and is defined like this:
 
   CFile getFile()
   {
 	return m_pFile;
   }
 
 So any idea about the reason of this error?


Pretend that delete is actually a function declared like this:

     void delete( inout void* val )
     {
         free( val );
         val = null;
     }

rvalues (ie. temporaries) cannot be passed to functions by reference 
because they don't have a concrete address (basically).  So you first 
have to store the result in an lvalue (a normal variable) before calling 
delete on it.


 The code compiles successfully if I replace the delete statement with:
 
   CFile tmpFile=pDoc.getFile();
   delete tmpFile;
 
 Another point, I guess D doesn't support "friend" classes or
 functions.. wouldn't they be useful sometimes?


They are.  But "friend" as a keyword isn't necessary in D because of the 
way visibility specifiers work.  "private" actually means "visible to 
everything in the current module" and "package" means "visible to 
everything in the current package."  And "protected" means "visible to 
everything in the current module (I think) and to derived classes."  So 
in essence, you can think of all class members as automatically being 
friends with one another.


Sean
Jan 05 2007
prev sibling next sibling parent reply Johan Granberg <lijat.meREM OVE.gmail.com> writes:
John Kiro wrote:


 Hello
 
 I have a problem with this statement:
 
   delete pDoc.getFile();
 
 It generates this error:
 
 "Error: ((cast(CDoc)(pDoc)).getFile)() is not an lvalue"


have you tried this "delete (pDoc.getFile());" ?
if I read the error mesage correctly the compiler thinks you want do delete
a function. (I have no compiler nearby so I cant test it)


 Another point, I guess D doesn't support "friend" classes or
 functions.. wouldn't they be useful sometimes?


Probably but I think they can cause trouble too, it comes down to weighting
pros and cons. Something to notice thou is that in D everything in the same
module has full access to all fields, as sort of implicit friend with
everything in the module.
Jan 05 2007
parent John Kiro <johnkirollos yahoo.com> writes:
== Quote from Johan Granberg (lijat.meREM OVE.gmail.com)'s article

 John Kiro wrote:

 Hello

 I have a problem with this statement:

   delete pDoc.getFile();

 It generates this error:

 "Error: ((cast(CDoc)(pDoc)).getFile)() is not an lvalue"


 have you tried this "delete (pDoc.getFile());" ?
 if I read the error mesage correctly the compiler thinks you want


do delete

 a function. (I have no compiler nearby so I cant test it)



It's rather the reason mentionned by Kirk & Sean: Attempting to use
the return of a function as a left-value.
So it seems that in this case the pointer invalidation done by
"delete" has lost its meaning, due to the workaround (defining a tmp
reference), because it will invalidate the tmp reference, not the
actual one (m_pFile).. but this is not be a big deal.. "delete" in
C++ doesn't touch the pointer anyway.



 Another point, I guess D doesn't support "friend" classes or
 functions.. wouldn't they be useful sometimes?


 Probably but I think they can cause trouble too, it comes down to


weighting

 pros and cons. Something to notice thou is that in D everything in


the same

 module has full access to all fields, as sort of implicit friend


with

 everything in the module.



Yes.. putting the 2 friends in the same module looks like a good
alternative for me.

Thanks friends
John
Jan 06 2007
prev sibling parent %u <u digitaldaemon.com> writes:
== Auszug aus John Kiro (johnkirollos yahoo.com)'s Artikel

 So any idea about the reason of this error?


You protected a pointer variable in a class with a getter.
Now you want to delete the object rooted in that pointer variable
from outside the class containing the pointer variable, thereby
leaving the value stored in that pointer variable unmodified.

This means you want that pointer variable in the class to have an
invalid address---to point to an object which does not exist
anymore.

The D compiler allows you to shoot in your foot if you really
really want to.
Jan 05 2007