Back to SYDE 223 lectures
Constructors
- A special kind of class member function
- Automatically called when object is instantiated
- Must be public
- If no constructor is defined by the user, the system will create a default constructor, which sets data members to their default values
- Difference from other regular member functions:
- They must have the same name as the class
- They cannot return a value; not even void
Constructor Demo
#include <iostream>
//declaration
class MonthAndDay {
private:
int month;
int day;
void checkDate(); //check if the date is valid.
public:
MonthAndDay(); // overload constructor with no argument condition //note: no return value not even void
MonthAndDay(int monthValue); //overload 1 int condition
MonthAndDay(int monthValue, int dayValue); //overload 2 int condition
void output(); //note: require return value such as void
};
//implementation
MonthAndDay::MonthAndDay() { //if no arg. set to Jan 1.
month = 1;
day = 1;
checkDate(); //don't need, we know it is valid.
}
MonthAndDay::MonthAndDay(int monthValue){
month = monthValue;
day = 1;
checkDate();
}
MonthAndDay::MonthAndDay(int monthValue, int dayValue) : month(monthValue), day(dayValue){
//can directly set the values after the : symbol
checkDate();
}
void MonthAndDay::checkDate(){
if((month < 1) || (month > 12)){
std::cout<<"Illegal month value!\n";
}
if((day < 1) || (day > 31)){
std::cout<<"Illegal day value!\n";
}
//of course, a better way is
//to check day depending on month, e.g., no Feb.30.
}
void MonthAndDay::output(){
std::cout<<"Month: "<<month<<", Day: "<<day<<std::endl;
}
//main function
int main() {
MonthAndDay date1; // call default constructor, don't use ()
MonthAndDay date2(5);
MonthAndDay date3(2,21);
date1.output();
date2.output();
date3.output();
//date1.MonthAndDay(); //ILLEGAL!
MonthAndDay date4(12,33);
MonthAndDay date5(-1,99);
return 0;
}
Why use constructors?
- Initialize member variables of an object. Simplify the procedure
- Add code to validate the entered initila values
- Other actions (ex. print to system log when an object is instantiated)
