Why does stringstream >> change value of target on failure?

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From this reference:

If extraction fails (e.g. if a letter was entered where a digit is expected), value is left unmodified and failbit is set (until C++11)

If extraction fails, zero is written to value and failbit is set. If extraction results in the value too large or too small to fit in value, std::numeric_limits::max() or std::numeric_limits::min() is written and failbit flag is set. (since C++11)

It seems that your compiler is compiling in C++11 mode, which changes the behavior.

The input operator uses the locale facet std::num_get whose get function invokes do_get. For C++11 it’s specified to use std::strtoll et. al. type of functions. Before C++11 it apparently used std::scanf style parsing (going by the reference, I don’t have access to the C++03 specification) to extract the numbers. The change in behavior is due to this change in parsing the input.

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