Why is integer output 0 when collecting string from input

Question

I'm new to C++ and am working through Stroustrup's Programming Principles and Practices. In section 3.3, he gives the following example:

#include <iostream>
#include <string>

int main() {
  std::cout << "Please enter your first name and age\n";
  std::string firstName;
  int age;
  std::cin >> firstName >> age;
  std::cout << "Hello, " << firstName << " (age " << age << ")\n";
}

As per the text, I entered '22 Carlos' and expect the output to be 22 followed by some random number or 0. Fine, I get 0.

I then initialise the variables and input the same '22 Carlos':

string firstName = "???";
int age = -1;

I expect the output to be 'Hello, 22 (age -1)', as stated in the text, but instead get 'Hello, 22 (age 0)'. Why?

Because your STL's implementation of `operator>>` is explicitly setting invalid int input to 0. That is allowed,though the behavior was undefined prior to C++11, but is guaranteed in C++11 and later. — Remy Lebeau, Jul 21 '18 at 07:54
Just check how to recover from stream failed state due to invalid inputs. — πάντα ῥεῖ, Jul 21 '18 at 07:56
@πάνταῥεῖ it's not a duplicate question as linked. I'm asking why the disparity between the book and the actual output, the linked question is asking if anyone uses the extraction operator. Totally different. — junglie85, Jul 21 '18 at 07:59
Whoever closed this question as a duplicate didn't read the answers to the duplicate, because NONE of them answer this question. So I'm reopening this question. — Remy Lebeau, Jul 21 '18 at 08:02
You can always refer to the [documentation](https://en.cppreference.com/w/cpp/io/basic_istream/operator_gtgt2) before asking here, sorry. — πάντα ῥεῖ, Jul 21 '18 at 08:03
@Remy Whoever wrote that question didn't bother to research. — πάντα ῥεῖ, Jul 21 '18 at 08:04
It's wrong, string cannot receive input from cin. Use getline; — Dennys Barreto, Jul 21 '18 at 17:09

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

The reason why you get age set to 0 is because (assuming you are on c++11 or higher):

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. (since C++11)

Just a side note, VC++ compiler was not following this standard until recently https://developercommunity.visualstudio.com/content/problem/285201/the-value-is-not-zeroed-after-failure-but-it-shoul.html

Why is integer output 0 when collecting string from input

1 Answers1