C++ Classes (I)

1. ECE230 Lectures Series C++ Classes (I) Ying Wu Electrical Engineering & Computer Science Northwestern University yingwu@ece.northwestern.edu

2. What shall we learn today? • What motivates the concept of C++ class? • What does it feature? • Is it good? • An example?

3. What have we learnt about C? • C language • Basic data types and syntax • Control structures • Function calls • What do you feel about C? • Powerful and flexible • Modular design • All based on function calls • The code are kinda hard to manage and reuse. 

4. C: function-based prog. • The way of thinking of a C programmer • thinks of actions (verb.) • is concern about how to produce the outputs given the inputs • makes all sorts of function calls • The way of doing of a C programmer • makes all sorts of small “tools” • selects right “tools” s/he built, and uses them to assembly the “data” into a program • “Tools” • A tool has a specific “caliber” (the argument list) • To use it, other people need to get a manual (the prototype) • Sometimes, it is hard to use w/o knowing the details of the implementation.

5. Disadvantages • It is ok if you only need to write and manage 1,000 lines of code. But what if your have 100,000 lines of code? • Code management and re-use are going to be difficult • C code only gives you a set of “tools” (functions), i.e., that actions to be taken • To build a “house” (the program), you need to prepare “brick”, “wood”, etc. (the data) • Obviously, the work is still tremendous!

6. Another Solution? • Is there another solution? • Yes! • Instead of providing small “tools”, • Why do I provide those “pre-built units” for you to get a “house” done? • A “hard-wood” floor • A “bedroom” • A “kitchen” • A “garage” • … • That would make life much easier!

7. The Change … • Changing the way of thinking! • Let’s re-think about our project in terms of “objects”, instead of “actions”. • What “objects” do we need if I want to implement a “MiniMatlab” system? • A command line interpreter • A system variable database • A arithmetic/logic unit • Matrix • Why don’t we put “data” and “functions” together to make “packages”? • Yes, that sounds a great idea!

8. C++: OOP • Object-oriented programming (OOP) • Encapsulates data (attributes) and functions (behavior) into packages called classes • Information hiding • Implementation details are hidden within the classes themselves • You, as an end user, don’t need to know the details of implementation. Just use it and enjoy! • Classes • Classes are the standard unit of programming • A class is like a blueprint – reusable • Note: we differentiate “class” and “object” • Objects are instantiated (created) from the class • For example, a house is an instance of a “blueprint class”

9. Class = Data + Functions • A class is a blueprint of a package • It consists of • Data members • Describe the attributes of a concept • Member functions • Describe the behavior of the data • An object is an instantiation of a class • A class is abstract • An object is real • To use it, you only need to know the “interface” • Some members are accessible, but some aren’t.

10. Example: “variable” • How do you describe a “variable”, for example? • “name”? • “value”? • Set a name? • Obtain the name? • Retrieve the value? • Set the value?

11. CVariable class CVariable { double m_dValue; char* m_sName; public: // constructors and destructors CVariable(); CVariable(const char*name, const double& v = 0.0); ~CVariable(); CVariable(const CVariable& var); // copy constructor const CVariable& operator=(const CVariable& var); // overload = // getting and setting double Value() { return m_dValue; }; char* Name() const { return m_sName; }; void SetValue(const double& v) { m_dValue = v; }; bool SetName(const char* name); };

12. Easy to use! void main() { CVariable a; CVariable b(“var_2”, 10.9); a.SetName(“var_1”); a.SetValue(b.Value() + 1.1); cout << a.Name() << a.Value() << endl; cout << b.Name() << b.Value() << endl; }

13. Example: “varDB” • How do you describe a variable DB? • A record? • Size of the DB? • Create and initialize a DB? • Add a record? • Display the DB? • Search the DB? • Let’s put them together!

14. CVarDB #define MAX_SIZE_DB 100 class CVarDB { CVariable m_pDB[MAX_SIZE_DB]; int m_nSize; // size of the database public: // constructors and destructors CVarDB(); ~CVarDB(){}; // interfaces void Init(); // return a valid ptr if found, else a NULL CVariable* Search(const char*name); // return a ptr of the new one, else a NULL CVariable* CreateANewVar(const char*name); void Dump(); };

15. Life is good! void main() { CVarDB mydb; mydb.Dump(); CVariable *tmpV; tmpV = mydb.CreateANewVar(“var_1”); tmpV->SetValue(10.8); if(mydb.Search(“var_2”)!=NULL){ cout << “found!” << endl; } tmpV = mydb.Search(“ans”); tmpV->SetValue(0.8); mydb.Dump(); } Even w/o looking at the implementation, I can use these class easily!

16. void CVarDB::Init() { m_nSize = 1; m_pDB[0].SetName("ans"); } CVariable* CVarDB::Search(const char* name) { CVariable *pVar = NULL; for(int i=0; i<m_nSize; i++){ if(!strcmp(m_pDB[i].Name(), name)){ pVar = &(m_pDB[i]); break; } } return pVar; }

17. CVariable* CVarDB::CreateANewVar(const char*name) { CVariable *pVar = NULL; if(m_nSize < SIZE_DB){ m_nSize ++; m_pDB[m_nSize-1].SetName(name); m_pDB[m_nSize-1].SetValue(0.0); pVar = &(m_pDB[m_nSize-1]); } return pVar; } void CVarDB::Dump() { cout.setf(ios::left, ios::adjustfield); for(int i=0; i<m_nSize; i++){ cout << " " << setw(20) << m_pDB[i].Name() << setw(15) << m_pDB[i].Value() << endl; } }

18. A Comparison • Let’s compare my C implementation (in Lecture 13) and my C++ implementation of the VarDB!