check count_if function. ch is a bool value, and when ch is true, the function would work. Combined with switchcase it lets you to count the number of a e i o u.
if you are confused about the structure [](){}, check out lambda in c++.

can someone explain how it works? espeically the

[](const char ch){
  switch(ch) {
          case 'a':
          case'e':
          case'i':
          case'o':
          case'u':
              return true;
          default:
              return false;
}