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<span id="Indexed-Assignment-Optimization-1"></span><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 <code>subsasgn</code> methods. Imagine the following
call to <code>subsasgn</code>
</p>
<div class="example">
<pre class="example">ss = substruct ("()", {1});
x = subsasgn (x, ss, 1);
</pre></div>
<p>where the corresponding method looking like this:
</p>
<div class="example">
<pre class="example">function x = subsasgn (x, ss, val)
…
x.myfield (ss.subs{1}) = val;
endfunction
</pre></div>
<p>The problem is that on entry to the <code>subsasgn</code> 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. In other words, a constant-time
operation is degraded to linear-time one. This may be a real problem for user
classes that intrinsically store large arrays.
</p>
<p>To partially solve the problem Octave uses a special optimization for
user-defined <code>subsasgn</code> methods coded as m-files. When the method gets
called as a result of the built-in assignment syntax (not a direct
<code>subsasgn</code> call as shown above), i.e., <code>x(1) = 1</code><!-- /@w -->, <b>AND</b> if
the <code>subsasgn</code> method is declared with identical input and output
arguments, as 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>
<p>It is important to understand the implications that this optimization brings.
Since no extra copy of <code>x</code> will exist in the caller’s scope, it is
<em>solely</em> the callee’s responsibility to not leave <code>x</code> in an invalid
state if an error occurs during 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>
<p>Since this optimization may change the way code works (especially if badly
written), a function <code>optimize_subsasgn_calls</code> is provided to
control it. This feature is enabled by default. Another way to avoid
the optimization is to declare subsasgn methods with different output
and input arguments like this:
</p>
<div class="example">
<pre class="example">function y = subsasgn (x, ss, val)
…
endfunction
</pre></div>
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