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<h4 class="subsection">34.3.2 Indexed Assignment Optimization</h4>
<p>Octave's ubiquitous lazily-copied pass-by-value semantics implies
a problem for performance of user-defined subsasgn methods. Imagine
a call to subsasgn:
<pre class="example"> ss = substruct ("()",{1});
x = subsasgn (x, ss, 1);
</pre>
<p class="noindent">and the corresponding method looking like this:
<pre class="example"> function x = subsasgn (x, ss, val)
...
x.myfield(ss.subs{1}) = val;
endfunction
</pre>
<p>The problem is that on entry to the subsasgn method, <code>x</code> is still
referenced from the caller's scope, which means that the method will
first need to unshare (copy) <code>x</code> and <code>x.myfield</code> before performing
the assignment. Upon completing the call, unless an error occurs,
the result is immediately assigned to <code>x</code> in the caller's scope,
so that the previous value of <code>x.myfield</code> is forgotten. Hence, the
Octave language implies a copy of N elements (N being the size of
<code>x.myfield</code>), where modifying just a single element would actually
suffice, i.e., degrades a constant-time operation to linear-time one.
This may be a real problem for user classes that intrinsically store large
arrays.
<p>To partially solve the problem, Octave uses a special optimization for
user-defined subsasgn methods coded as m-files. When the method
gets called as a result of the built-in assignment syntax (not direct subsasgn
call as shown above), i.e.
<pre class="example"> x(1) = 1;
</pre>
<p><b>AND</b> if the subsasgn method is declared with identical input and output argument,
like in the example above, then Octave will ignore the copy of <code>x</code> inside
the caller's scope; therefore, any changes made to <code>x</code> during the method
execution will directly affect the caller's copy as well.
This allows, for instance, defining a polynomial class where modifying a single
element takes constant time.
<p>It is important to understand the implications that this optimization brings.
Since no extra copy of <code>x</code> in the caller's scope will exist, it is
<em>solely</em> the callee's responsibility to not leave <code>x</code> in an invalid
state if an error occurs throughout the execution. Also, if the method
partially changes <code>x</code> and then errors out, the changes <em>will</em> affect
<code>x</code> in the caller's scope. Deleting or completely replacing <code>x</code>
inside subsasgn will not do anything, however, only indexed assignments matter.
<p>Since this optimization may change the way code works (especially if badly
written), a built-in variable <code>optimize_subsasgn_calls</code> is provided to
control it. It is on by default. Another option to avoid the effect is to
declare subsasgn methods with different output and input arguments, like this:
<pre class="example"> function y = subsasgn (x, ss, val)
...
endfunction
</pre>
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