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| <TITLE>A Simple Extractor and Inserter for the Example</TITLE> |
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| <H2>32.3 A Simple Extractor and Inserter for the Example</H2> |
| <P>Following the read and write conventions of iostreams, we would insert and extract the date object like this:</P> |
| |
| <UL><PRE> |
| date birthday(2,6,1952); |
| std::cout << birthday << '\n'; |
| </PRE></UL> |
| <P>or </P> |
| |
| <UL><PRE> |
| date aDate; |
| |
| std::cout << '\n' << "Please, enter a date (day month year)" |
| << '\n'; |
| std::cin >> aDate; |
| std::cout << aDate << '\n'; |
| </PRE></UL> |
| <A NAME="idx781"><!></A> |
| <P>For the next step, we would implement shift operators as inserters and extractors for <SAMP>date</SAMP> objects. Here is an extractor for class <SAMP>date</SAMP>:</P> |
| |
| <UL><PRE> |
| template<class charT, class Traits> |
| std::basic_istream<charT, Traits>& //1 |
| operator>> (std::basic_istream<charT,Traits>& is, //2 |
| date& dat) //3 |
| { |
| is >> dat.tm_date.tm_mday; //4 |
| is >> dat.tm_date.tm_mon; |
| is >> dat.tm_date.tm_year; |
| return is; //5 |
| } |
| </PRE></UL> |
| <TABLE CELLPADDING="3"> |
| |
| <TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The returned value for extractors (and inserters) is a reference to the stream, so that several extractions can be done in one expression. |
| <TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The first parameter usually is the stream from which the data shall be extracted. |
| <TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The second parameter is a reference, or alternatively a pointer, to an object of the user-defined type. |
| <TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>In order to allow access to private data of the date class, the extractor must be declared as a friend function in class <SAMP>date</SAMP>. |
| <TR VALIGN="top"><TD><SAMP>//5</SAMP></TD><TD>The return value is the stream from which the data was extracted. |
| </TABLE> |
| <A NAME="idx782"><!></A> |
| <P>As the extractor accesses private data members of class date, it must be declared as a friend function to class date. The same holds for the inserter. Here's a more complete version of class <SAMP>date</SAMP>:</P> |
| |
| <UL><PRE> |
| class date { |
| public: |
| date(int d, int m, int y); |
| date(tm t); |
| date(); |
| // more constructors and useful member functions |
| |
| private: |
| std::tm tm_date; |
| |
| template<class charT, Traits> |
| friend std::basic_istream<charT, Traits> |
| &operator >>(std::basic_istream<charT, Traits >& is, date& dat); |
| |
| template<class charT, Traits> |
| friend std::basic_ostream<charT, Traits> |
| &operator <<(std::basic_ostream<charT, Traits >& os, |
| const date& dat); |
| }; |
| </PRE></UL> |
| <A NAME="idx783"><!></A> |
| <P>The inserter can be built analogously, as shown below. The only difference is that you would hand over a <I>constant</I> reference to a date object, because the inserter is not supposed to modify the object it prints.</P> |
| |
| <UL><PRE> |
| template<class charT, class Traits> |
| std::basic_ostream<charT, Traits>& |
| operator << (std::basic_ostream<charT, Traits >& os, |
| const date& dat) |
| { |
| os << dat.tm_date.tm_mon << '-'; |
| os << dat.tm_date.tm_mday << '-'; |
| os << dat.tm_date.tm_year ; |
| return os; |
| } |
| </PRE></UL> |
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