How to verify total digits and ensure characters are not entered?

Question

in C++

string phone_number

If asking for a 10 digit phone number for example, how would you ensure 10 digits are entered?  Also, how would you ensure no characters were input by mistake.  I am also curious how to ensure the user doesn't enter past the prompt by mistake (leaving it empty).

Answer 1

#include <iostream>
#include <string>
#include <cctype>

using namespace std;

bool isValidPhone(const string& s) {
    if (s.empty()) return false;
    if (s.size() != 10) return false;

    for (char ch : s) {
        if (!isdigit(static_cast<unsigned char>(ch)))
            return false;
    }
    return true;
}

int main() {
    string phone_number;

    while (true) {
        cout << "Enter a 10-digit phone number: ";
        getline(cin, phone_number);   // allows empty input too (we can detect it)

        if (isValidPhone(phone_number)) {
            break;
        }

        cout << "Invalid input. Please enter exactly 10 digits (0-9) only.\n";
    }

    cout << "Valid phone number: " << phone_number << endl;
    return 0;
}

Answer 2

#include <iostream>
#include <string>
#include <cctype>

bool isValidPhoneNumber(const std::string& phone)
{
    // Check length
    if (phone.length() != 10)
        return false;

    // Check all characters are digits
    for (char c : phone)
    {
        if (!std::isdigit(c))
            return false;
    }

    return true;
}

int main()
{
    std::string phone_number;

    while (true)
    {
        std::cout << "Enter a 10-digit phone number: ";
        std::getline(std::cin, phone_number);

        // Prevent empty input (user just pressed Enter)
        if (phone_number.empty())
        {
            std::cout << "Input cannot be empty.\n";
            continue;
        }

        if (isValidPhoneNumber(phone_number))
        {
            break;  // valid input
        }

        std::cout << "Invalid phone number. Please enter exactly 10 digits.\n";
    }

    std::cout << "Phone number accepted: " << phone_number << "\n";
}