string accepting only numbers

[Cp9]

How I can check if credit card numbers contains only of numbers?

# include <iostream>
# include <string>
# include <iomanip>
using namespace std;


int main (){

    string creditCard;

    do{
    cout << "Enter credit card number:\t";
    cin  >> creditCard;

    for (int i = 0; i < 16; i++)
        {

        if (creditCard.length < 16 && creditCard.length > 16)

        cout << "\nInvalid credit card number.Please try again:";

        }
    }while(true);
return 0;
}

[Repeater]

Examine each char in the string. If the char you are examining is not a number, then the string is not just numbers.

[lastchance]

if ( creditCard.find_first_not_of( "0123456789" ) == string::npos ) cout <<"Valid";