[Octave-bug-tracker] [bug #59500] sub-classes of octave_base_value aren't assignable

[anonymous]

Follow-up Comment #6, bug #59500 (project octave):

An assignment operator is needed to allow copying the data contents of an
object and disallow copying the reference count. The proposed assignment
operator for octave_base_value in comment #3 does that. Setting the assignment
operator of octave_base_value to default won't compile because it tries to
assign to the 'refcount' that isn't by definition assignable.
 
 One may argue that if someone defines 'myfunction' as:
 

 octave_value_list
myfunction (const octave_value_list& lst, int n = 0)
{
  static octave_value val = args(0);
  return ovl ();
}


ans uses  `*static_cast<octave_scalar*> (lst(0).internal_rep ()) = double(i);`
it leads to an unwanted result and in the next iterations the static variable
is changed. One can prevent such behavior by defining a second octave_value:
 

octave_value_list lst = ovl (octave_value (0.0));

octave_value tmp(0.0);

for (int i = 0; i < 1000000; i++)
  {
    *static_cast<octave_scalar*> (tmp.internal_rep ()) = double(i);
    
    lst(0) = tmp;
    
    myfunction (lst);
  }


Another solution is checking the reference count:


octave_value_list lst = ovl (octave_value (0.0));

for (int i = 0; i < 1000000; i++)
  {
    if (lst(0).get_count () > 1)
      lst(0) = double(i);
    else
      *static_cast<octave_scalar*> (lst(0).internal_rep ()) = double(i);
      
    myfunction (lst);
  }


However if the decision is keeping the current behavior I suggest to make
octave_value behave faster for lightweight objects (possibly embedding small
objects directly in octave_value). If it is OK I may upload a patch that
implements such structure. This strategy may be extended to small arrays and
lists.

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