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    copied!<p>The "obvious" solution is to use a manipulator to install a custom <code>std::num_put&lt;char&gt;</code> facet which just formats <code>int</code>s as desired.</p> <p>The above statement may be a bit cryptic although it entirely describes the solution. Below is the code to actually implement the logic. The first ingredient is a special <code>std::num_put&lt;char&gt;</code> <em>facet</em> which is just a class derived from <code>std::num_put&lt;char&gt;</code> and overriding one of its <code>virtual</code> functions. The used facet is a filtering facet which looks at a flag stored with the stream (using <code>iword()</code>) to determine whether it should change the behavior or not. Here is the code:</p> <pre><code>class num_put : public std::num_put&lt;char&gt; { std::locale loc_; static int index() { static int rc(std::ios_base::xalloc()); return rc; } friend std::ostream&amp; twodigits(std::ostream&amp;); friend std::ostream&amp; notwodigits(std::ostream&amp;); public: num_put(std::locale loc): loc_(loc) {} iter_type do_put(iter_type to, std::ios_base&amp; fmt, char fill, long value) const { if (fmt.iword(index())) { fmt.width(2); return std::use_facet&lt;std::num_put&lt;char&gt; &gt;(this-&gt;loc_) .put(to, fmt, '0', value); } else { return std::use_facet&lt;std::num_put&lt;char&gt; &gt;(this-&gt;loc_) .put(to, fmt, fill, value); } } }; </code></pre> <p>The main part is the <code>do_put()</code> member function which decides how the value needs to be formatted: If the flag in <code>fmt.iword(index())</code> is non-zero, it sets the width to <code>2</code> and calls the formatting function with a fill character of <code>0</code>. The width is going to be reset anyway and the fill character doesn't get stored with the stream, i.e., there is no need for any clean-up.</p> <p>Normally, the code would probably live in a separate translation unit and it wouldn't be declared in a header. The only functions really declared in a header would be <code>twodigits()</code> and <code>notwodigits()</code> which are made <code>friend</code>s in this case to provide access to the <code>index()</code> member function. The <code>index()</code> member function just allocates an index usable with <code>std::ios_base::iword()</code> when called the time and it then just returns this index. The <em>manipulators</em> <code>twodigits()</code> and <code>notwodigits()</code> primarily set this index. If the <code>num_put</code> facet isn't installed for the stream <code>twodigits()</code> also installs the facet:</p> <pre><code>std::ostream&amp; twodigits(std::ostream&amp; out) { if (!dynamic_cast&lt;num_put const*&gt;( &amp;std::use_facet&lt;std::num_put&lt;char&gt; &gt;(out.getloc()))) { out.imbue(std::locale(out.getloc(), new num_put(out.getloc()))); } out.iword(num_put::index()) = true; return out; } std::ostream&amp; notwodigits(std::ostream&amp; out) { out.iword(num_put::index()) = false; return out; } </code></pre> <p>The <code>twodigits()</code> manipulator allocates the <code>num_put</code> facet using <code>new num_put(out.getloc())</code>. It doesn't require any clean-up because installing a facet in a <code>std::locale</code> object does the necessary clean-up. The original <code>std::locale</code> of the stream is accessed using <code>out.getloc()</code>. It is changed by the facet. In theory the <code>notwodigits</code> could restore the original <code>std::locale</code> instead of using a flag. However, <code>imbue()</code> can be a relatively expensive operation and using a flag should be a lot cheaper. Of course, if there are lots of similar formatting flags, things may become different...</p> <p>To demonstrate the use of the manipulators there is a simple test program below. It sets up the formatting flag <code>twodigits</code> twice to verify that facet is only created once (it would be a bit silly to create a chain of <code>std::locale</code>s to pass through the formatting:</p> <pre><code>int main() { std::cout &lt;&lt; "some-int='" &lt;&lt; 1 &lt;&lt; "' " &lt;&lt; twodigits &lt;&lt; '\n' &lt;&lt; "two-digits1='" &lt;&lt; 1 &lt;&lt; "' " &lt;&lt; "two-digits2='" &lt;&lt; 2 &lt;&lt; "' " &lt;&lt; "two-digits3='" &lt;&lt; 3 &lt;&lt; "' " &lt;&lt; notwodigits &lt;&lt; '\n' &lt;&lt; "some-int='" &lt;&lt; 1 &lt;&lt; "' " &lt;&lt; twodigits &lt;&lt; '\n' &lt;&lt; "two-digits4='" &lt;&lt; 4 &lt;&lt; "' " &lt;&lt; '\n'; } </code></pre>
 

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