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When calling num_get<>::get(), information about conversion errors is returned to the caller through the ios_base::iostate& argument. The C++ Standard, section [facet.num.get.virtuals], paragraph 3 states, in part 3 [ISO/IEC 14882-2014], in part, states the following:
If the conversion function fails to convert the entire field, or if the field represents a value outside the range of representable values,
ios_base::failbitis assigned toerr.
Always explicitly check the error state of a conversion from string to a numeric value (or handle the related exception, if applicable) instead of assuming the conversion results in a valid value. This rule is in addition to ERR34-C. Detect errors when converting a string to a number, which bans the use of conversion functions that do not perform conversion validation such as std::atoi() and std::scanf() from the C Standard Library.
Noncompliant Code Example
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In this compliant solution, each converted value read from the standard input stream is tested for validity before reading the next value in the sequence, allowing error recovery on a per-value basis. It checks std::istream::fail() to see if the failure bit was set due to a conversion failure or whether the bad bit was set due to a loss of integrity with the stream object. If a failure condition is encountered, it is cleared on the input stream and then characters are read and discarded until a ' ' (space) character occurs. Note that the The error handling in this case only works if a space character is what delimits the two numeric values to be converted.
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