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Object Array

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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;

Graphviz diagram

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