Returning values from class functions

[AKat]

I recently picked up Deitel's C++ How to Program 7e. I'm currently working on one of the exercises in the book and I was curious on a different way of returning a value from one of the class functions. Currently I have the following line in main:

myHeartRate.getTargetHeartRate(( (myHeartRate.getMaximumHeartRate (myHeartRate.getAge(month, day, year))) ));

The related class functions code:

int HeartRates::getAge(int month, int day, int year)
{
   if ( month <= birthMonth )
      return ( year - birthYear - 1 );

   if ( month == birthMonth )
      if ( day < birthDay )
         return ( year - birthYear - 1 );

   if ( month > birthMonth )
      return ( year - birthYear );
}

int HeartRates::getMaximumHeartRate( int age )
{
   return 220 - age ;
}

void HeartRates::getTargetHeartRate( int maxHeartRate )
{
   cout << maxHeartRate * .5 << " - " << maxHeartRate * .85 << endl;
}

Is it possible to have the getTargetHeartRate return 2 integer values rather than having it spit out a cout? The program works as it is, but I guess what I'm trying to ask, is there a "best practice" method to use. One of the problems from learning straight out of a book is that there's no one to ask questions.

I tried changing getTargetHearRate to:

int HeartRates::getTargetHeartRate( int maxHeartRate )
{
   return maxHeartRate * .5;
   return maxHeartRate * .85;
}

But it only returned one value. Thanks in advance.

[Zeillinger]

What about returning an array? Something like...

int* HeartRates::getTargetHeartRate( int maxHeartRate )
{
	int result[2];
	result[0] = maxHeartRate * .5;
	result[1] = maxHeartRate * .85;
	return result;
}

// ... Later on
int *rates = my.heartRate.getTaegetHeartRate(...);

[Disch]

Zeillinger's approach won't work. You'd be returning a bad pointer.

If you want to have multiple outputs, you have a few options:

Option 1)
Put the data in a struct

struct HeartRate
{
  double low;
  double high;
};

HeartRate HeartRates::getTargetHeartRate( int maxHeartRate )
{
  HeartRate r;
  r.low = maxHeartRate * 0.5;
  r.high = maxHeartRate * 0.85;
  return r;
}

Option 2)
Pass the output parameters "by reference":

void HeartRates::getTargetHeartRate(int maxHeartRate, double& low, double& high)
{
  low = maxHeartRate * 0.5;
  high = maxHeartRate * 0.85;
}

//...

double low, high;
heart.getTargetHeartRate(whatever, low, high);  // this function assigns low, high 

[Zeillinger]

Disch wrote:

Zeillinger's approach won't work. You'd be returning a bad pointer.

As I'm learning as well, can you elaborate? Is the deleting them the problem?

[Disch]

Yes.

You're not returning an array, you're returning a pointer. That pointer points to 'result'.

However, as soon as the function exits, 'result' goes out of scope and is destroyed. Anything that points to it after it is destroyed is a bad pointer.

[Zeillinger]

If result was allocated using new (i.e. int *result = new int[2];) would it make a difference? I think there's still problem with deleting the pointer but I better make sure :S

[Disch]

yes that would work, but then you'd have to remember to delete it later or else you leak memory.

I don't recommend passing ownership like that when it can be reasonably avoided. Passing memory ownership between function calls is messy and hard to manage.

KISS: 1 block of memory -> 1 owner

[Zeillinger]

Thanks, that piece of advice was really helpful.

[AKat]

The book I'm going through hasn't described structures yet, so I'm looking through some online references to it.

Arrays, pointers, and references. The last book I looked through before putting it down was horrible at explaining it. This one describes them in later chapters and was much better at explaining it. Generally I go through the book (any computer-related book) until my head spins and I acknowledge the fact that I tried to go through to quickly, then I start over. It seems to help in letting me retain the information that I digested as well as making me feel good knowing "Hey! I can do that"!

Would it also be correct in assuming that it's better to declare any variables in use by a class within the class itself rather than passing it off through main?