Thread: catch()

I am confused about the bolded part.

Why do we have to throw Overflow(), Underflow() and Bad_size() and not just throw Overflow, Underflow... without the parantheses - why do we have to throw a "function" and not the type of the object itself?

And same goes for structures, not just classes.

Code:

#include <cstdlib>
#include <iostream>

using namespace std;

class Stack {
      char *v;
      int top;
      int max_size;
      public:
             class Overflow { };
             class Underflow { };
             class Bad_Size { };
             struct ov {
             };
             
             Stack(int s);
             ~Stack();
             
             void push(char c);
             char pop();
};

Stack :: Stack(int s)
{
      top = 0;
      if(s > 10000) 
         throw Bad_Size();
      max_size = s;
      v = new char[s];
}

Stack :: ~Stack()
{
      delete [] v;
}

void Stack :: push(char c)
{
     if(top == max_size - 1)
        throw Overflow();
     v[top++] = c;
}

char Stack :: pop()
{
     if(top == 0)
        throw Underflow();
     return v[--top];
}

int main(int argc, char *argv[])
{
      Stack :: Bad_Size();
      try {
          Stack sp(10000);
          int i;
          for(i = 0; i <= 5; i++)
             sp.push('c');
      }
      catch(Stack :: Underflow) {
          cout << "underflow!" << '\n';
      }
      catch(Stack :: Overflow) {
          cout << "overflow!" << '\n';
      }
      catch(Stack :: Bad_Size) {
          cout << "bad size!" << '\n';
      }
      Stack :: Bad_Size();
      getchar();
      return 0;
}