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   4     </p><p>A few extensions and nods to backwards-compatibility have been made with
   5    containers.  Those dealing with older SGI-style allocators are dealt with
   6    elsewhere.  The remaining ones all deal with bits:
   7 </p><p>The old pre-standard <code class="code">bit_vector</code> class is present for
   8    backwards compatibility.  It is simply a typedef for the
   9    <code class="code">vector&lt;bool&gt;</code> specialization.
  10 </p><p>The <code class="code">bitset</code> class has a number of extensions, described in the
  11    rest of this item.  First, we'll mention that this implementation of
  12    <code class="code">bitset&lt;N&gt;</code> is specialized for cases where N number of
  13    bits will fit into a single word of storage.  If your choice of N is
  14    within that range (&lt;=32 on i686-pc-linux-gnu, for example), then all
  15    of the operations will be faster.
  16 </p><p>There are
  17    versions of single-bit test, set, reset, and flip member functions which
  18    do no range-checking.  If we call them member functions of an instantiation
  19    of "bitset&lt;N&gt;," then their names and signatures are:
  20 </p><pre class="programlisting">
  21    bitset&lt;N&gt;&amp;   _Unchecked_set   (size_t pos);
  22    bitset&lt;N&gt;&amp;   _Unchecked_set   (size_t pos, int val);
  23    bitset&lt;N&gt;&amp;   _Unchecked_reset (size_t pos);
  24    bitset&lt;N&gt;&amp;   _Unchecked_flip  (size_t pos);
  25    bool         _Unchecked_test  (size_t pos);
  26    </pre><p>Note that these may in fact be removed in the future, although we have
  27    no present plans to do so (and there doesn't seem to be any immediate
  28    reason to).
  29 </p><p>The semantics of member function <code class="code">operator[]</code> are not specified
  30    in the C++ standard.  A long-standing defect report calls for sensible
  31    obvious semantics, which are already implemented here:  <code class="code">op[]</code>
  32    on a const bitset returns a bool, and for a non-const bitset returns a
  33    <code class="code">reference</code> (a nested type).  However, this implementation does
  34    no range-checking on the index argument, which is in keeping with other
  35    containers' <code class="code">op[]</code> requirements.  The defect report's proposed
  36    resolution calls for range-checking to be done.  We'll just wait and see...
  37 </p><p>Finally, two additional searching functions have been added.  They return
  38    the index of the first "on" bit, and the index of the first
  39    "on" bit that is after <code class="code">prev</code>, respectively:
  40 </p><pre class="programlisting">
  41    size_t _Find_first() const;
  42    size_t _Find_next (size_t prev) const;</pre><p>The same caveat given for the _Unchecked_* functions applies here also.
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