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Testing the State of an I/O Stream |
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In Chapter 4, we talked about the concept of input and output streams in C++. We introduced the data types istream, ostream, ifstream, and ofstream. We said that any of the following can cause an input stream to enter the fail state: |
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An attempt to read beyond the end of a file |
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An attempt to open a nonexistent file for input |
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C++ provides a way in which to determine if a stream is in the fail state. In a logical expression, you simply use the name of the stream variable as if it were a Boolean variable: |
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if (cin)
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if ( !inFile )
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When you do this, you are said to be testing the state of the stream. The result of the test is either a nonzero value (meaning the last I/O operation on that stream succeeded) or zero (meaning the last I/O operation failed). |
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Conceptually, you want to think of a stream variable in a logical expression as being a Boolean variable with a value TRUE (the stream state is okay) or FALSE (the state isn't okay). |
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Testing the State of a Stream The act of using a C++ stream variable in a logical expression as if it were a Boolean variable; the result is nonzero (TRUE) if the last I/O operation on that stream succeeded, and zero (FALSE) otherwise. |
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In an If statement, the way you phrase the logical expression depends on what you want the then-clause to do. The statement |
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