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

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?59500>

                 Summary: sub-classes of  octave_base_value aren't assignable
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Sun 22 Nov 2020 03:24:21 PM UTC
                Category: Interpreter
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Regression
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 6.0.92
         Discussion Lock: Any
        Operating System: Any

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Details:

I recently noticed that all sub-classes of `octave_base_value` aren't
copy-assignable. For example:


octave_scalar a(1.0);
octave_scalar b(2.0);
b = a;


The example works on Octave 5.2.0 but in 6.0.92 doesn't work.
The compiler says that the `count` member of `octave_base_value` is of class
`octave::refcount<octave_idx_type>` and the copy assignment operator of
`octave::refcount<octave_idx_type>` is explicitly deleted. Because of that the
copy assignment operator of `octave_base_value` is also implicitly deleted and
because `octave_scalar` is a sub-class of `octave_base_value` and it doesn't
defined its own copy assignment operator its default assignment operator is
also deleted.

Consider a typical signature of builtin functions:


octave_value_list 
myfunction (const octave_value_list& lst, int n = 0)
{}


Now I want to use this function in a loop. One way is:


octave_value_list lst = ovl (octave_value (0.0));

for (int i = 0; i < 1000000; i++)
  {
    lst(0) = double(i);
    myfunction (lst);
  }


The statement `lst(0) = double(i);` calls the constructor of
`octave_value::octave_value(double d)` and it internally calls `new 
octave_scalar(d)`. So creating a scalar value needs at least a memory
allocation.
A second way to doing the task is:


octave_value_list lst = ovl (octave_value (0.0));
for (int i = 0; i < 1000000; i++)
  {
    *static_cast<octave_scalar*> (lst(0).internal_rep ()) = double(i);

    myfunction (lst);
  }


I prefer and use the second method because it doesn't need extra 1000000
memory allocations.
Unfortunately the second method doesn't work in new alpha 6.0.92 version.

Many Thanks!






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