How can I achieve this kind of relationship (inheritance, composition, something else)?

Question

I would like to set up a foundation of classes for an application, two of which are person and student. A person may or may not be a student and a student is always a person. The fact that a student “is a” person led me to try inheritance, but I can't see how to make it work in the case where I have a DAO that returns an instance of person and I then want to determine if that person is a student and call student related methods for it.

class Person {
    private $_firstName;

    public function isStudent() {
        // figure out if this person is a student
        return true; // (or false)
    }
}

class Student extends Person {
    private $_gpa;

    public function getGpa() {
        // do something to retrieve this student's gpa
        return 4.0; // (or whatever it is)
    }
}

class SomeDaoThatReturnsPersonInstances {
    public function find() {
        return new Person();
    }
}

$myPerson = SomeDaoThatReturnsPersonInstances::find();

if($myPerson->isStudent()) {
    echo 'My person\'s GPA is: ', $myPerson->getGpa();
}

This obviously doesn't work, but what is the best way to achieve this effect? Composition doesn't sond right in my mind because a person does not “have a” student. I'm not looking for a solution necessarily but maybe just a term or phrase to search for. Since I'm not really sure what to call what I'm trying to do, I haven't had much luck. Thank you!

I could, yes. In the Student class, isStudent() would always be true. If I have an instance of the base Person class, isStudent() may or may not be true. — Tim, Apr 03 '10 at 17:03

score 0 · Answer 1 · answered Apr 03 '10 at 13:40

<?php
class Person {
    #Can check to see if a person is a student outside the class with use of the variable
    #if ($Person->isStudentVar) {}
    #Or with the function
    #if ($Person->isStdentFunc()) {}

    public $isStudentVar = FALSE;  

    public function isStudentFunc() {
        return FALSE;
    }
}

class Student extends Person {
    #This class overrides the default settings set by the Person Class.
    #Also makes use of a private variable that can not be read/modified outside the class

    private $isStudentVar = TRUE;  

    public function isStudentFunc() {
        return $this->isStudentVar;
    }

    public function mymethod() {
        #This method extends the functionality of Student
    }
}

$myPerson1 = new Person;
if($myPerson1->isStudentVar) { echo "Is a Student"; } else { echo "Is not a Student"; }
#Output: Is not a Student

$myPerson2 = new Student;
if($myPerson2->isStudentFunc()) { echo "Is a Student"; } else { echo "Is not a Student"; }
#Output: Is a Student
?>

I would choose one way and stick to it. Just deomonstrating various ideas and techniques.

Thanks for your reply. The isStudentFunc() in the base Person class won't always return false though. If it returns true, I would want to be able to do something like this: $myPerson1 = new Person(); if($myPerson1->isStudentFunc()) { $myPerson1->mymethod(); } — Tim, Apr 03 '10 at 17:02

How can I achieve this kind of relationship (inheritance, composition, something else)?

1 Answers1