Integer to String: Without using stoi library

I found this question in the book where it asks us to transfer an Int to a string. Without using stoi library so for example if x = 10, s="10" the code should handle negative numbers.

I found this solution in the book. I typed it in my compiler however it is giving the string of the first number only

so if x= 45, it is giving "4"

I do not understand this line s = '0' + x%10; to be able to fix the code. why is he adding '0' to the string. What is the best solution.

here is the code: I added comments to the parts I understand

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


void IntToString(int x);
int main()
{
    int num;
    cout << "Please enter a number" << endl;
    cin >> num;

    IntToString(num);
}

void IntToString(int x)
{
    bool isNegative = false;
    if(x < 0)         //if its negative make boolean true 
    {
        x = -x;
        isNegative = true;
    }
    string s;
    do
    {
        s = '0' + x%10;    //modulus for getting the last number
        x = x/10;   //shorten the number
    }while(x); 
    reverse(s.begin(), s.end()); //reverse the string since it starts from end

    if(isNegative)
        s = '-' + s;
    cout << s << endl;
}