Object Array

Motivation

Array of objects is an efficient method to model a list of records. Compared to other alternative methods to model multiple records, object array excels in many aspects.

1D Array of Objects

When modeling a list of N records with M fields (a N * M 2D table), there are several data structures to choose from:

Multiple arrays

• store data in M arrays, each with size N, same index means same record

  E.g. grades[i] and names[i] refers to the grade and name of a same student

  const int NUM_STUDENT = 100; // max number of students
  double grades[NUM_STUDENT];
  string names[NUM_STUDENT];
  
  names[4] = "John Smith";
  grades[4] = 100;
  cout << "The 5th student in the list " << names[4];
  cout << " has a grade of " << grades[4];
  

• old-fashion approach inherited from the C language

Array of objects

• Pre-allocate N empty objects using the default constructor

• Each object has M fields

• Local array example:

  // struct example
  struct StudentStruct {
    double grade;
    string name;
  };
  
  StudentStruct students[NUM_STUDENT];
  students[4].name = "John Smith";
  students[4].grade = 100;
  cout << "The 5th student in the list " << students[4].name;
  cout << " has a grade of " << students[4].grade;
  
  // class example
  class StudentClass {
    double grade;
    string name;
  public:
    StudentClass(): name(""), grade(0) {}
    StudentClass(string name, double grade): name(name), grade(grade) {}
    string getName() { return name;}
    double getGrade() { return grade;}
  };
  
  StudentClass students1[NUM_STUDENT];  // default constructor called 100 times
  students1[4] = StudentClass("John Smith", 100);  // disposable object assignment
  cout << "The 5th student in the list " << students1[4].getName() << " has a grade of " << students1[4].getGrade();
  

• To update the fields of the object after the initial allocation of empty objects, you can either

  1. use assignment or setters:

     // if name and grade fields are public
     students[4].name = "John Smith";
     students[4].grade = 100;
     
     // if name and grade fields are accessible only through setters
     students1[4].setName("John Smith");
     students1[4].setGrade(100);
     

  • fast, modifying existing objects

  • can be very verbose given large number of fields

  1. assign temporary object:

     students1[4] = Student("John Smith", 100);
     

  • overhead of creating and duplicating temporary objects

  • cleaner syntax

• Dynamic array example:

  StudentClass *students2;
  students2 = new StudentClass[NUM_STUDENT];
  students1[4] = StudentClass("John Smith", 100);  // temporary object assignment
  cout << "The 5th student in the list " << students1[4].getName() << " has a grade of " << students1[4].getGrade();
  delete [] students2;
  

Array of pointers to objects

• Pre-allocate N pointers

• Initialized to nullptr

• Use new operator to allocate objects as needed

• Each object has M fields

• Complex syntax than array of objects

• Local array example:

  StudentClass *students3[NUM_STUDENT];  // 100 pointers
  for (int i = 0; i < NUM_STUDENT; ++i)
    students3[i] = nullptr;
  students3[4] = new StudentClass("John Smith", 100);
  cout << "The 5th student in the list " << students3[4]->getName();
  cout << " has a grade of " << students3[4]->getGrade();
  
  // release memory
  for (int i = 0; i < NUM_STUDENT; ++i)
    delete students3[i];  // delete objects, no []
  

• Dynamic array example:

  StudentClass **students4;  // do not confuse with 2D array of objects
  students4 = new StudentClass *[NUM_STUDENT];  // 100 pointers
  for (int i = 0; i < NUM_STUDENT; ++i)
    students4[i] = nullptr;
  students4[4] = new StudentClass("John Smith", 100);
  cout << "The 5th student in the list " << students4[4]->getName();
  cout << " has a grade of " << students4[4]->getGrade();
  
  // release memory
  for (int i = 0; i < NUM_STUDENT; ++i)
    delete students4[i];  // delete objects, no []
  delete [] students4;
  

Pitfalls

• mismatched assignment:

  StudentClass students[10];
  students[4] = new StudentClass("Mary Johnson", 100);
  // should have no 'new' operator
  

• wrong member accessing operator:

  StudentClass *students[10];
  students[4] = new StudentClass("Mary Johnson", 100);
  cout << students[4].getName() << endl;
  // should use -> given students[4] as a pointer
  

• confused delete and delete []

• not checking nullptr before delete