Introduction to Records (structs)

Chapter 9

A struct is a data type that is a collection of a fixed number of components (called members), that are accessed by name.

• The members of a struct may be of different types.

• The syntax for defining a struct:

  struct StructName
  {
      dataType1 identifier1;
      dataType2 identifier2;
      …
      dataTypeN identiferN;
  }; //<- ends in a semicolon

A struct is a definition of a new data type, not a variable declaration. One must declare a variable of that type to use it. Remember, a data type is a set of possible values that can be stored in a variable with the matching type.

/**
 * Defining a new type called "Movie" that holds metadata of a movie.
 */
struct Movie
{
    string title;
    int year;
    string storyline;
    string genre;
    double rating;
    int reviewCount;
    bool isFavorite;
};

Just like we can create an integer variable with int value;, variables of our struct types can be declared (e.g., Move favMove;).

Movie favMove; // example of declaring a variable with our movie type

The variable favMovie after declaration. Move favMove;

A struct type must be defined before it can be used. Normally, we put the structs above the main and function prototypes. This is okay because we are defining a type, not a variable.

Accessing struct Members

Syntax to access a struct member: structVariableName.memberName

The dot (.) is called the member-access operator.

To initialize the members of favMovie:

// Initialize some of the members of this Movie variable.
favMovie.title = "Toy Story 3";
favMovie.year = 2010;
favMovie.isFavorite = true;

// Display the movie's title
cout << favMovie.title << endl; // "Toy Story 3"

The variable favMovie after initializing some of its members.

Assignment

• The value of one struct variable can be assigned to another struct variable of the same type using an assignment statement (an aggregate operation).

• The following statement copies the contents of favMovie to oldFavMovie (both are Movie variables as defined above).

  oldFavMovie = favMovie;

  It is equivalent to:

  oldFavMovie.title = favMovie.title;
  oldFavMovie.year = favMovie.year;
  oldFavMovie.storyline = favMovie.storyline;
  oldFavMovie.genre = favMovie.genre;
  oldFavMovie.rating = favMovie.rating;
  oldFavMovie.reviewCount = favMovie.reviewCount;
  oldFavMovie.isFavorite = favMovie.isFavorite;

The values of oldFavMovie are copied to favMovie, so any future changes to members of oldFavMovie, will NOT be reflected in favMovie.

• You can also assign values to structs using an ordered list in {}

  // Set the values of the oldFavMovie in order (title, year, …, isFavorite).
  oldFavMovie = { "Moana", 2016, "Ocean leads girl on journey", "animation",
      7.6, 264776, false };

  Design Note: This type of aggregate initialization may introduce bugs if you add new members to your struct type.

Comparison (using Relational Operators)

• Two struct variables must be compared member-wise. No aggregate relational operations are given.

• To compare the values of movie1 and movie2:

  if (movie1.title == movie2.title && movie2.year == movie2.year && …)

Input and Output

• No aggregate input/output operations on a struct variable

• Data in a struct variable must be read or written one member at a time

• Example: output favMovie contents

  cout << "Title: " << favMovie.title << '\n' // Toy Story 3
      << "Year: " << favMovie.year << '\n'    // 2010
      << "Storyline: " << favMovie.storyline << '\n'
      << "Genre: " << favMovie.genre << '\n'
      << "Rating: " << favMovie.rating << '\n'
      << "Review Count: " << favMovie.reviewCount << '\n'
      << "Favorite: $" << (favMovie.isFavorite ? "yes" : "no") << endl; // yes

Structs as Function Parameters and Return Values

Try the following code from the video.

/*
 * Introduction to records. Creating our own structured data types!
 * by Dr. Hayes
 */
#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

/**
 * A data type representing a student's info.
 */
struct Student
{
	string firstName;
	string lastName;
	double gpa;
};

/**
 * Display the information about the student.
 */
void print(const Student& stud);
 
/**
 * Get a student's info from the user.
 */
void getStudent(Student& stud);

/**
 * Get a student's info from the user.
 */
Student getStudent();

int main()
{
	Student stud1 = {"James", "Kirk", 3.5};
	Student stud2;

	stud1 = getStudent();
	getStudent(stud2);

	if (stud1.firstName == stud2.firstName
		&& stud1.lastName == stud2.lastName 
		&& stud1.gpa == stud2.gpa)
	{
		cout << "The two students are the same." << endl;
	}

	cout << setprecision(1) << fixed;
	print(stud1);
	print(stud2);

	return 0;
}

void print(const Student& stud)
{
	cout << stud.firstName << " " << stud.lastName 
		<< " has a GPA of " << stud.gpa << endl;
}

void getStudent(Student& stud)
{
	cout << "Enter your first name: ";
	cin >> stud.firstName;

	cout << "Enter your last name: ";
	cin >> stud.lastName;

	cout << "Enter your current GPA: ";
	cin >> stud.gpa;
}

Student getStudent()
{
	Student stud;

	getStudent(stud);

	return stud;
}

Arrays versus structs

A comparison of the operations that may be performed on arrays and structs.

Aggregate Operation	Array	struct
Arithmetic
List Initialization	✓	✓
Assignment		✓
Input / Output	*Strings only
Comparison
Parameter Passing	*By Reference Only	✓
Function Returning a Value		✓

Summary

• struct: a collection of a fixed number of components (called members) that are accessed by name.
• Components can be of different types.
• struct is a reserved word.
• No memory is allocated for a struct type. A memory location is reserved when a variable is declared.
• The dot operator (.) is the member-access operator
  • Used to access members of a struct
• The only built-in operations on a struct are the assignment and member access
• Neither arithmetic nor relational operations are allowed on structs
• A struct can be passed by value or reference
• A function can return a value of type struct