C++ inline function program

using namespace std;

int check_digit(char);

int main() {
  char c;
   
  cout << "Enter a character\n";
  cin >> c;                                
   
  if ( check_digit(c) ) {
    cout << c << " is a digit.\n";
  }
  else {
    cout << c << " isn't a digit.\n";
  }
 
  return 0;
}

inline int check_digit(char c) {
  if ( c >= '0' && c <= '9' ) {
    return 1;
  }
  else {
    return 0;
  }
}

using namespace std;

class programming {
  char c;
 
  public:
    void input();
    void output();
};

inline void programming::input() {
  cin >> c;
}

void programming::output() {
  cout << c << endl;
}

int main() {
  programming object;
 
  object.input();
  object.output();
 
  return 0;
}