Compound Structures

Section 15.6 Compound Structures

Once we have defined a structures, we can use it to define other structures. For example, given this date structure:

struct Date {
  int month;
  int day;
  int year;
};

We can define a new structure representing a student that includes a date:

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struct Student {
  string name;
  string email;
  Date enrollmentDate;
};

This says that “a Student as a name and email that are strings and an enrollmentDate which is a Date.” To diagram what a Student variable named s1 might look like, we would draw something like this:

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A student struct that contains a date struct — Figure 15.6.1.
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In this diagram, the Student struct is shown as a box with three parts. Two parts are the name and email strings. The third part is a Date struct, which is itself a box with three parts. When we say s1.enrollmentDate we are talking about the entire date struct that is a part of that student. To talk about the members of that struct, we need to use the dot operator again. For example, to access the month of a student’s enrollment date, we would write s1.enrollmentDate.month.

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This program demonstrates creating and using our compound data type:

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Listing 15.6.2.

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#include <iostream>
#include <string>
using namespace std;

struct Date {
    int month;
    int day;
    int year;
};

struct Student {
    string name;
    string email;
    Date enrollmentDate;
};

void printDate(const Date& d) {
    cout << d.month << "/" << d.day << "/" << d.year << endl;
}

void printStudent(const Student& s) {
    cout << "Name: " << s.name << endl;
    cout << "Email: " << s.email << endl;
    cout << "Enrollment Date: ";
    // pass just the enrollmentDate of this student to printDate
    printDate(s.enrollmentDate);
}

int main() {
    // Create a Date object
    Date d1 = {9, 14, 2023};
    printDate(d1);

// Create a Student object using the Date object
    Student s1 = {
      "Rene Parker",
      "rpark3@somewhere.edu",
      d1
    };

cout << "Student 1: " << s1.name << ", "
         << s1.email << ", "
         << s1.enrollmentDate.month << "/"
         << s1.enrollmentDate.day << "/"
         << s1.enrollmentDate.year << endl;

// Create a Student object using an initializer list as date
    Student s2 = {
      "Mario Martinez",
      "mmart7@somewhere.edu",
      { 10, 1, 2023 }
    };

cout << "Student 2: ";
    printStudent(s2);
}

Some things to note:

Student s1’s enrollmentDate is initialized using Date d1. It gets a copy of that date - 9/14/2023.

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Lines 41-45 print student s1’s members one by one. Note that printing the date requires accessing each part individually. We can’t just say cout << s1.enrollmentDate as the compiler doesn’t know how to print a Date. We need use s1.enrollmentDate.month which specifies a single int. That the compiler knows what to do with.

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Student s2’s enrollmentDate is initialized using an initializer list (lines 48-52). So we have a list for the date inside the list for the student.

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We print s2 using the printStudent function. It uses s.enrollmentDate to access the entire Date struct for that student and pass it to printDate.

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The memory diagram for the state at the end of main would look something like this:

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A Date d1 and Students s1 and s2 — Figure 15.6.3.
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Checkpoint 15.6.1.

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Checkpoint 15.6.2.

Hint.

You should use the list syntax - { } - to specify the date.

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