What c++ do for `my_map[i] = my_map.size()`?
<pre><code>std::map&lt;int, int&gt; my_map;
my_map[0] = my_map.size();
</code></pre>

<p>Then, will <code>my_map[0]</code> be <code>0</code> or <code>1</code>, or undefined?</p>
