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| <TITLE>An Example</TITLE> |
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| <H2>37.2 An Example</H2> |
| <A NAME="idx897"><!></A> |
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| <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& io, int index) |
| { |
| if (e == std::ios_base::imbue_event) |
| std::cout << "imbue called" << std::endl; |
| else if (e == std::ios_base::erase_event) |
| std::cout << "stream destroyed" << std::endl; |
| else |
| std::cout << "cpyfmt called" << 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<>::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<>::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 26.9.2</A> for an example using callback functions with a user-defined stream inserter.</P> |
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