文字⇒数値

#include <bits/stdc++.h>
using namespace std;
int main() {
string S;
cin >> S;
int a = (S[1] - '0');
a *= 2;
cout << a << endl;
}

数値⇒文字

  string S = "hello";
  int a = 3;
  cout << S + to_string(a) << endl;

桁数を求める

  int digit = 0;
  while(N != 0){
   N = N / 10;
   digit++;
  }