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| <H2>32.6 Patterns for Extractors and Inserters of User-Defined Types</H2> |
| <A NAME="idx796"><!></A> |
| <P>Here is the pattern for an extractor:</P> |
| |
| <UL><PRE> |
| template<class charT, class Traits> |
| std::basic_istream<charT, Traits>& |
| operator>> (std::basic_istream<charT, |
| Traits>& is, UserDefinedType& x) |
| { |
| std::ios_base::iostate err = 0; |
| |
| // Create a sentry object (may set error bits or throw failure) |
| typename basic_istream<charT, Traits>::sentry ipfx(is); |
| |
| try { |
| |
| // Proceed with I/O only if sentry is okay. |
| |
| if(ipfx) { |
| // Typically you access the stream's locale or buffer. |
| // Don't call other stream I/O functions. |
| |
| // Add state bits to the err variable if necessary, |
| // for example: |
| // if (...) |
| // err |= std::ios_base::failbit; |
| } |
| } |
| catch(...) { |
| bool flag = false; |
| try { |
| // set failbit (may throw failure) |
| is.setstate(std::ios_base::failbit); |
| } |
| catch(...) { flag = true; } |
| |
| // Re-throw original exception (thrown from facet or buffer) |
| if (flag) |
| throw; |
| } |
| |
| // As the last step, set error bits (may throw failure) |
| if (err) |
| is.setstate(err); |
| |
| return is; |
| } |
| </PRE></UL> |
| <A NAME="idx797"><!></A> |
| <P>Similarly, the pattern for the inserter looks like this:</P> |
| |
| <UL><PRE> |
| template<class charT, class Traits> |
| std::basic_ostream<charT, Traits>& |
| operator<<(std::basic_ostream<charT, Traits >& os, |
| const UserDefinedType& x) |
| { |
| std::ios_base::iostate err = 0; |
| |
| // Create a sentry object (may set error bits or throw failure) |
| typename basic_ostream<charT, Traits>::sentry opfx(os); |
| |
| try { |
| // Proceed with I/O only if sentry is okay |
| if (opfx) { |
| // Typically you access the stream's locale or buffer. |
| // Don't call other stream I/O functions |
| |
| // Add state bits to the err variable if necessary, |
| // for example: |
| // if (...) |
| // err |= std::ios_base::failbit; |
| |
| // Reset the field width after usage, that is, |
| // os.width(0); |
| } |
| } //try |
| catch(...) { |
| bool flag = false; |
| try { |
| // set failbit (may throw failure) |
| os.setstate(std::ios_base::failbit); |
| } |
| catch(...) { |
| // Catch exception, do not rethrow |
| flag = true; |
| } |
| // Rethrow original exception (from facet or buffer) |
| if (flag) |
| throw; |
| } |
| |
| // As the last step, set error bits (may throw failure) |
| if (err) |
| os.setstate(err); |
| |
| return os; |
| } |
| </PRE></UL> |
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