CODEWITHSUNDEEP
c++functionclass
user2809437
user2809437

Reputation: 492

understanding c++ class and function calls

I'm trying to understand this example we did in class but having some trouble...

For a class Time, an instance of this class is made up of hrs, mins secs

So 

Time labStart(10,30,0);
Time labEnd (12,20,0);

 (labEnd-labStart).printTime() //I'm not concerned with the printTime function

const Time Time::operator - (const Time& t2) const {

    int borrow=0;
    int s=secs-t2.secs;

    if (s<0) {
     s+=60;
     borrow=1;
    }

    int m=mins-t2.mins2-borrow;
     if (m<0) {
     m+=60;
     borrow=1;
    }
    else 
      borrow=0;

    int h= hrs-t2.hrs-borrow;
     if (h<0) {
     h+=24;

     Time tmp=Time(h,m,s);
     return tmp;
}

So if we are passing both labEnd and labStart, and I was told (labEnd-labStart) ~ labEnd.operator-(labStart)

I dont understand how and where labEnd's variables are considered? In the function above only one parameter of Time is passed in, labStart, so the t2.mins t2.sec accounts for labStarts mins and secs,(30 mins and 0 sec respectively) however where is labEnd's variables (12,20,0)?? (instance variables hours, mins, secs)??

Upvotes: 1

Views: 53

Answers (2)

Barmar
Barmar

Reputation: 780842

labEnd - labStart

is equivalent to:

labEnd.operator -(labStart)

So labEnd is this in the member function, and its member variables can be accessed as if they were ordinary variables.

Upvotes: 0

John Kugelman
John Kugelman

Reputation: 361585

In your function this is a pointer to &labEnd. The bare secs, mins, and hrs mentions have an implicit this-> in front of them. If you write out the this->'s explicitly the three variable declarations become:

int s = this->secs - t2.secs;
int m = this->mins - t2.mins - borrow;
int h = this->hrs  - t2.hrs  - borrow;

Upvotes: 2

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