You can use try/catch to catch exceptions. The std::string at function will throw a out_of_range error if you try to use an illegal index:

   std::string S = "Hello World";
   try
   {
      std::string::size_type i = 0;
      while(1)
      {
         std::cout << S.at(i++);
      }
   }
   catch(std::out_of_range e)
   {
      std::cout << " The end!" << std::endl;
      std::cout << "What:" << e.what() << std::endl;
   }

Notice that the [ ] operator does not throw an exception, the result is undefined if you try to use an illegal index.

You can define you own exception:

class MyException
{
public:
   MyException(std::string aWhat) : what(aWhat)
   {}
   std::string what;
};

To use it:

void Print(int x)
{
   if(x > 10)
      throw MyException("At the end");

   std::cout << x << std::endl;
}
int main()
{
   try
   {
      int i = 0;
      while(1)
         Print(i++);
   }
   catch(MyException Except)
   {
      std::cout << Except.what << std::endl;
   }
}