Answer:
Here the code is given as follows,
Explanation:
#include <iostream>
#include <string>
using namespace std;
void Vowels(string userString);
int main()
{
    string userString;
    //to get string from user
    cout << "Please enter a string: ";
    getline(cin,userString,'\n');
    Vowels(userString);
    return 0;
}
void Vowels(string userString)
{
    char currentChar;
    //variables to hold the number of instances of each vowel
    int a = 0, e = 0, i = 0, o = 0, u = 0;  
    for (int x = 0; x < userString.length(); x++)  
    {
        currentChar = userString.at(x);
        switch (currentChar)  
        {
            case 'a':
                a += 1;
                break;
            case 'e':
                e += 1;
                break;
            case 'i':
                i += 1;
                break;
            case 'o':
                o += 1;
                break;
            case 'u':
                u += 1;
                break;
            default:
                break;
        }
    }
    // to print no of times a vowels appears in the string
    cout << "Out of the " << userString.length() << " characters you entered." << endl;
    cout << "Letter a = " << a << " times" << endl;
    cout << "Letter e = " << e << " times" << endl;
    cout << "Letter i = " << i << " times" << endl;
    cout << "Letter o = " << o << " times" << endl;
    cout << "Letter u = " << u << " times" << endl;
}
Hence the code and Output.
Please enter a string
Out of the 16 characters you entered.
Letter a = 2 times
Letter e = 1 times
Letter i = 0 times
Letter o = 1 times
Letter u = 0 times.