Why does cin, expecting an int, change the corresponding int variable to zero in case of invalid input? [duplicate]

Answer 1

With the compiler I tested against (gcc 4.8.4), the value is set to zero (for the 'abc' input) regardless of which version of the standard I compile against. Note also that it is set to minimum/maximum values if you provide a valid integer that is outside of the supported range of the variable that you are assigning to.

A more important point though, is that ignoring error flags is a recipe for disaster. Once you have an error in the input, any subsequent input is suspect (without hacks like ignore()). Situations like this are a good candidate for using exception handling.

Here's how I might implement what you are trying to do (but, again, considering the case of multiple inputs, recovering from errors is a messy business):

cin.exceptions( ~std::ios::goodbit );
try { cin >> age; }
catch ( std::ios::failure const & )
   {
   age=-1;
   cin.clear();
   cin.ignore(999,'\n');
   }

or without exeptions:

cin >> age;
if ( cin.fail() ) age=-1, cin.clear(), cin.ignore(999,'\n');

See here for similar questions:

• Why does stringstream >> change value of target on failure?
• istream behavior change in C++ upon failure

Here are up-to-date docs for the operator in question:

• http://www.cplusplus.com/reference/istream/istream/operator%3E%3E/
• http://en.cppreference.com/w/cpp/io/basic_istream/operator_gtgt

Answer 2

The behavior you want to observe changed in 2011. Until then:

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

But since C++11:

If extraction fails, zero is written to value and failbit is set. [...]

(From cppr.)