[C++] Chap 07 - Constructors and Other tools

Constructors

• Initialization of class object (Called when objects are declared)
• Must have the same name as the class
• No output type. (NOT even void)
• Placed in the public section
• Values in parentheses are passed to arguments to constructor.

DayOfYear date1(7, 4), date2(5, 5);
// Called when declaring class object
date1.DayOfYear(7, 4); //Very ILLEGAL. Cannot call constructor like other member functions

Constructor definition

DayOfYear::DayOfYear(int monthValue, int dayValue) {
{
	month = monthValue;
	day = dayValue;
}
// First DayOfYear : class name
// Second DayOfYear : constructor name
// Note no return type

DayOfYear::DayOfYear(int monthValue, int dayValue) : month(monthValue), day(dayValue)
{
// Body doesn't have to be empty
// Can validate the data!
// Ensure only appropriate data is assigned to class private member variables
}

• Constructers can also be overloaded.
  • Constructor which doesn’t have any parameter, is called default constructor

class DayOfYear {
public :
	DayOfYear(int monthValue, int dayValue);
private :
	const int month;
	const int day;
};

DayOfYear::DayOfYear(int monthValue, int dayValue) {
{
	month = monthValue;
	day = dayValue;
}
// since month and day are const member variables, 
// cannot declare like month = monthValue; 
// because the 'month' should be initialized when created

Explicit Constructor Calls

holiday = DayOfYear(5, 5);
// Create and initialize an anonymous object
// and assign it to holyday.

• Quite different from standard member function call (since it has no return type)

Default Constructors

• If no constructors defined, a default constructor doing nothing will be automatically created
• Otherwise, if you want to declare like SampleClass myVar;, need to define a default constructor

class SampleClass
{
	public:
		SampleClass(int param1, double param2);
		SampleClass();
		void doStuff();
	private:
		int data1;
		double data2;
};
int main() {
	SampleClass myvar, myvar2(1, 2.0);
	// if you want to define myvar, needto define default constructor

Class member variable

• Call back to member object’s constructor

Holiday::Holiday() : date(1, 1), parkingEnforcement(false){
}
//date(1, 1) is a invocations of constructors from the class 'DayOfYear'

More tools

Const modifier for functions

• Mark a funciton with a const modifier so as to notify it should not change the values of the object
• If const is used for a class type, then every member function should have const modifier.

void foo(const BankAccount& ba)
{
	bar;
	ba.output(); // ILLEAGL if output does not have the const modifier
}

Three uses of Const

1. const double pi=3.1416; Global variable
2. void display(const Student& a); Parameter
3. void show() const; Function modifier

Inline Member functions

• Function definition can be defined in the class definition
• Efficient for short definitions
• Can only be called in the same file as the one in which it is defined

Maybe not included..

Static member variables

• All object of class share only one copy.
• On object changes it, all can see the change
• Similar to global var. But only for the objects of the class

Static member functions

• Cannot access the object’s non-static variable data
• Only access static data/function of class
• Nonstatic member function can also access static variables of the class

Server::getTurn(); // Class::func(); format

• Every static variable must be initialized outside the class function : only once!