input stream with string

Question

If I type john when prompted for a char, the while statement will loop 4 times, one for each letter of john, before it asks again for user input.

Why does this program do not allow me insert more input before the whole 4 chars of john are consumed ? I would expect it to discard the 3 remaining letters of the string john and asked me for more input on the second loop.

The whole example can be found at page 44 of Bjarne Stroustrup The C++ Programming Language 4th edition.

#include <iostream>
using namespace std;

bool question() {
  while (true) {
    cout << "Continue ?\n";
    char answer = 0;
    cin >> answer;
    cout << "answer: " << answer << endl;
  }
  return false;
}

int main () {
  cout << question() << endl;
}

The output becomes:

Continue ?
john
answer: j
Continue ?
answer: o
Continue ?
answer: h
Continue ?
answer: n
Continue ?

Chris · Accepted Answer · 2023-01-11 16:21:42Z

1

You may be wondering why you're not being allowed to enter a character at each prompt. You have entered four characters into the input stream, so your loop runs four times to consume all of that input.

If you only want to use the first character in the input, you may want to get an entire line and work on just the first character.

#include <iostream>
#include <string>

bool question() {
  while (true) {
    std::cout << "Continue ?\n";
    std::string line;
    std::getline(std::cin, line);
    std::cout << "answer: " << line[0] << endl;
  }
  return false;
}

Of course, you should also check that an empty line was not entered, which may be as simple as checking if line[0] is not '\0'.

edited Jan 11, 2023 at 16:21

answered Jan 11, 2023 at 16:10

Chris

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4 Comments

NathanOliver Over a year ago

FWIW since C++11 line[0] will always work. If the string is empty then you will get a null terminator.

Ben Voigt Over a year ago

@NathanOliver: IIRC it worked prior to C++11 as well.... The address of the terminator didn't have to be contiguous with the rest of the string unless c_str() had been called, but just reading it using operator[] was perfectly fine.

NathanOliver Over a year ago

@BenVoigt In C++08/03 it was legal for a const std::string but UB otherwise. source: 21.3.4 basic_string element access

Ben Voigt Over a year ago

@NathanOliver: Ahh yes, can't have programmers overwriting the shared terminator storage. Still, that doesn't require const std::string, it just requires a const std::string&

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