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32 lines
1.8 KiB
Text
32 lines
1.8 KiB
Text
The bf(C++) I/O library was used well before exceptions were available in
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bf(C++). Hence, normally the classes of the iostream library do not throw
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exceptions. However, it is possible to modify that behavior using the
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ti(ios::exceptions) member function. This function has two overloaded
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versions:
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itemization(
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ithtq(exceptions (function))(ios::iostate exceptions())
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(hi(iostate)this member returns the i(state flags) for which the
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stream will throw exceptions;)
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itt(void exceptions(ios::iostate state))
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quote(hi(iostate)this member causes the stream to throw an exception
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when state tt(state) is observed.)
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)
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In the I/O library, exceptions are objects of the class
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hi(failure class) tt(ios::failure), derived from hi(exception class)
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tt(ios::exception). A tt(std::string const &message) may be specified when
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defining a tt(failure) object. Its message may then be retrieved using its
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hi(what) tt(virtual char const *what() const) member.
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Exceptions should be used in exceptional circumstances. Therefore, we
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think it is questionable to have stream objects throw exceptions for fairly
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normal situations like endOfFile(). Using exceptions to handle input errors
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might be defensible (e.g., in situations where input errors should not occur
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and imply a corrupted file) but often aborting the program with an appropriate
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error message would probably be the more appropriate action. As an example
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consider the following interactive program using exceptions to catch incorrect
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input:
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verbinclude(examples/ioexceptions.cc)
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By default, exceptions raised from within tt(ostream) objects are caught by
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these objects, which set their tt(ios::badbit) as a result. See also the
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link(paragraph on this issue)(STRBUFLABEL) in section ref(STREAMBUF).
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