Testing input data

This is so simple I am embarrassed. Well not really being a first semester C++ student.

What is the code to perform the following:

I want to test 'answer' to be sure it contains ONLY digits....
Any help will greatly be appreciated!!
Thank you



Ask user to input a value
//start of snippet
int answer;
cout << "What is the answer? ";
cin >> answer;
//end of snippet

Code:

---

#include <iostream>
#include <cstring>
#include <string>
#include <cctype>
using namespace std;

int main()
{
    string answer;
    cout << "What is the answer? ";
    cin >> answer;

    for (int i = 0; i < answer.length(); i++)
    {
        if (!isdigit(answer[i])
        {
            cout << "Invalid answer" << endl;
            break;
        }
    }

---

Thank you very much!

Your assistance is appreciated...