The Single Responsibility Principle says a class should be cohesive and have only one primary responsibility. Clear responsibilities make classes easier to understand, use, and maintain. section 2.3.1
Apply the principle by:
- Separating stored data from collection management: Book stores attributes, while Catalogue adds books and searches them. section 2.3.1
- Questioning classes that combine unrelated operations; an automobile class covering driving, maintenance, and cleaning is overly complex. section 1.4.2
- Balancing useful separation against unnecessary class proliferation, since good design can add flexibility but also create complexity. section 1.4.2
Book catalogue
A Book holds title, last name, and first name. Catalogue owns the book list, adds books, and finds matches, including searches with empty fields as don't-cares. Test data demonstrates exact matches, partial matches, and no matches. section 2.3.1
Listing 1.3 (Program 1.2 Changes): Automobile.h (poorly designed)
class AutomobileApp
{
public:
void accelerate();
void adjust_headlights();
void apply_brakes();
void change_oil();
void change_tires();
void check_brakes();
void check_tires();
void rotate_tires();
void shut_off_engine();
void start_engine();
void tuneup_engine();
void turn_left();
void turn_right();
void vacuum_car();
void wash_car();
void wax_car();
private:
Brake brakes[4];
Engine engine;
Oil engine_oil;
Direction heading;
Headlight headlights[4];
int speed;
Soap soap;
Tire tires[4];
Vacuum vacuum_cleaner;
CarWax wax;
};
Inspect the poorly designed AutomobileApp listing to see how unrelated behaviors and state accumulate in one class. section 1.4.2
