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

Why Does stringstream >> Alter Target Value on Extraction Failure After C 11?

Stroustrup's "TC PL" states that a target variable's value should remain unchanged if an istream or ostream operation fails. However, this contradicts observed behavior with stringstream:

#include <iostream>
#include <sstream>

int main() {
  std::stringstream ss;
  int v = 123;
  ss << "The quick brown fox.";
  if (ss >> v) {
    std::cout << "Unexpected success in reading a word into an int!\n";
  }
  std::cout << "After extraction failure: " << v << "\n";
  return 1;
}

This code prints "After extraction failure: 0," despite Stroustrup's claim.

C 11 Behavior Modification

This contradiction stems from a change in stringstream behavior after C 11. Before C 11, stringstream used a scanf-style parsing method, leaving the target value unchanged on failure. However, since C 11, it employs std::strtoll, which writes a zero to the value variable and sets failbit on extraction failure.

This change aligns with the revised C 11 behavior of istream and ostream operations:

• On failure, the target value is set to 0 or the appropriate extreme value.
• failbit is set to indicate the failure.

Conclusion

While Stroustrup's quote accurately describes the behavior of istream and ostream operations prior to C 11, stringstream's post-C 11 behavior deviates from this description due to the adoption of a different extraction mechanism. In C 11 and beyond, it's essential to be aware of this altered behavior when using stringstream operations.

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