Teaching Notes: Module 05

Key Learning Objectives

• Exception handling (try/catch/throw)
• Custom exception classes
• Nested exception classes
• Exception-safe design

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Exercise 00: Bureaucrat

Key Concepts

• Grade 1 = highest, 150 = lowest
• Increment = grade goes DOWN (more senior)
• Throw exceptions for invalid grades

Common Mistakes

1. Grade logic backwards

   void Bureaucrat::incrementGrade() {
       _grade++;  // WRONG - increment means better = lower number
       _grade--;  // CORRECT
   }

2. Exception class not inheriting std::exception

   class GradeTooHighException : public std::exception {
       const char* what() const throw() {
           return "Grade is too high";
       }
   };

3. Not making name const

   • const std::string _name; - can’t change after construction

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Exercise 01: Form

Key Design

• Form has grades required to sign AND execute
• Bureaucrat signs Form, Form validates grade

Common Mistakes

1. Checking wrong direction

   // Remember: lower number = better
   if (bureaucrat.getGrade() > _gradeToSign)
       throw GradeTooLowException();

2. signForm() output format

   • Success: <name> signed <form>
   • Failure: <name> couldn't sign <form> because <reason>

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Exercise 02: Abstract Form (AForm)

Key Changes

• Form -> AForm (abstract)
• Add execute(Bureaucrat const&) const
• Concrete forms: Shrubbery, Robotomy, Presidential

Execution Requirements

1. Form must be signed
2. Executor grade must be high enough

Guiding Questions

• “Where should execution requirements be checked - base or derived?”
• “Why might it be better to check in the base class?”

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Exercise 03: Intern

The Factory Pattern

AForm* Intern::makeForm(const std::string& name, const std::string& target) {
    // Use function pointers or map, NOT if/else forest
}

Common Mistakes

1. Using if/else chain

   • Subject explicitly forbids this
   • Use array of function pointers

2. Memory management

   • Intern creates form, caller must delete

Clean Implementation

typedef AForm* (*FormCreator)(const std::string&);

AForm* createShrubbery(const std::string& t) {
    return new ShrubberyCreationForm(t);
}
// ... other creators ...

AForm* Intern::makeForm(const std::string& name, const std::string& target) {
    std::string names[] = {"shrubbery creation", "robotomy request", "presidential pardon"};
    FormCreator creators[] = {createShrubbery, createRobotomy, createPresidential};

    for (int i = 0; i < 3; i++) {
        if (name == names[i]) {
            std::cout << "Intern creates " << name << std::endl;
            return creators[i](target);
        }
    }
    std::cerr << "Form not found: " << name << std::endl;
    return NULL;
}