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<TITLE>An Example</TITLE>
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<H2>37.2 An Example</H2>
<A NAME="idx897"><!></A>
<A NAME="idx898"><!></A>
<P>A program can register a callback function on a particular stream as follows:</P>

<UL><PRE>
// Callback function
void show_event(std::ios_base::event e,
                std::iosbase&amp; io, int index)
{
  if (e == std::ios_base::imbue_event)
    std::cout &lt;&lt; "imbue called" &lt;&lt; std::endl;
  else if (e == std::ios_base::erase_event)
    std::cout &lt;&lt; "stream destroyed" &lt;&lt; std::endl;
  else
    std::cout &lt;&lt; "cpyfmt called" &lt;&lt; std::endl;
}


std::ostringstream s;
s.register_callback(show_event,0);         //1
s.imbue(std::locale::global());            //2
</PRE></UL>
<TABLE CELLPADDING="3">

<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The function <SAMP>show_event</SAMP> is now called with either <SAMP>std::ios_base::erase_event</SAMP>, <SAMP>std::ios_base::imbue_event</SAMP>, or <SAMP>std::ios_base::copyfmt_event</SAMP> as the first argument, depending on whether the destruction of the stream, the imbuing of a new locale, or a call to <SAMP>std::basic_ios&lt;&gt;::copyfmt()</SAMP> initiated the callback.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>This causes <SAMP>show_event</SAMP> to be called. The first argument is <SAMP>std::ios_base::imbue_event</SAMP>; the second argument is a reference to the stream where the event occurred, which is <SAMP>s</SAMP> in this case; and the third argument is always the index provided in the call to <SAMP>std::ios_base&lt;&gt;::register_callback()</SAMP>, which is <SAMP>0</SAMP> in this case.
<BR><BR>If more than one function is registered, functions are called in the opposite order of registration. 
</TABLE>
<P>Please refer to <A HREF="26-9.html#2692">Section&nbsp;26.9.2</A> for an example using callback functions with a user-defined stream inserter.</P>

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