cin in C++ trying to assign string values to an int variable?

Question

I'm working through Bjarne Stroustrup's Programming - Principles and Practice Using C++ and came to the following example:

#include "std_lib_facilities.h"

int main() {
    /* Name and Age input */
    cout << "Please enter your first name and age\n";
    string first_name = "???";
    int age = -1.0;
    cin >> first_name >> age;
    cout << "Hello," << first_name << "(age " << (age * 12) << " months)\n";
    return 0;
 }

If you run the program and input Carlos 22, it will correctly output Hello, Carlos (age 22). However, if you put in 22 Carlos, he says that the output should be Hello, 22 (age -1) because since "Carlos isn't an integer... it will not be read". However, when I run it, it returns Hello, 22 (age 0) which seems like it's assigning a garbage value to it. I'm curious as to why this is happening, as the book implies that unless you input an integer, it won't try to assign anything to the age variable. I ran the code with breakpoints and confirmed the value of age changes from -1 to 0 once the non-integer input has been entered.

Am I doing something wrong? Or is this an oddity due to the fact I'm building it through Xcode and whatever compiler comes included with that?

Answer 1

If you follow the trail of calls from the call

cin >> age;

you end up with a call to std::strtol. The return value of strtol is:

• If no conversion can be performed, ​0​ is returned.

Check the status of cin after the calls to make sure that all the extractions were successful.

cin >> first_name >> age;
if ( cin )
{
   // Extraction was successful.
   // Use the data.
}
else
{
   // Extraction was not successful.
   // Deal with the error.
}