Why Does `stringstream >>` Zero Out the Target Variable on Extraction Failure in C 11 and Later?

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Why Does stringstream >> Change the Value of Target on Failure?

In the quoted passage from Stroustrup's "TC PL, 3rd Edition," the author states that the value of v should remain unchanged if the extraction operation fails. However, an example contradicts this statement, as v gets zeroed out when the stringstream >> v operation fails.

Contradictory Behavior Explanation

The key to understanding this apparent contradiction lies in the different behavior of the extraction operator >> before and after C 11.

Before C 11

Prior to C 11, >> used a scanf-style parsing mechanism. When extraction failed, it left the value of v unchanged and set the failbit flag.

Since C 11

With C 11, >> switched to using the std::num_get facet for parsing input. As per the updated specification, if extraction fails:

• For basic types like int, 0 is written to v and the failbit flag is set.
• For types with overflow protection, such as std::numeric_limits::max() or std::numeric_limits::min() is written to v and the failbit flag is set.

Example Output Explanation

In the provided example:

• The value of v is initially 123.
• After attempting to read a word into v (which fails), the value of v is set to 0.
• The stringstream's rdstate() indicates that the failbit is set, confirming the extraction failure.

This behavior is consistent with the updated behavior of >> introduced in C 11.

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