Review

• For a single value, use basic data types: int, double, bool, etc.
• Arrays can store sequences of values of the same type; therefore, they are called homogeneous.
• Multidimensional arrays can be used to store data for checkerboards, 2-d games, 3-d graphics, etc, where all data is again of the same type. Consider, for example:

int rubiks_cube[NFACES][NROWS][NCOLS];

This structure can store data for a rubik's cube, where each color is represented as an integer. But what if you have data that is not of the same type? This is called heterogeneous data.

Introducing struct

A struct is an aggregate data structure, like an array: it keeps values together. However, a struct can store elements of different types – it is thus heterogeneous. Also, its elements are named – C++ calls them member variables.

Just as there are lots of real-world examples of arrays, there are many examples of heterogeneous collections: A wallet might contain paper money, coins, pictures, credit cards. A jewelry box usually has different sized compartments for earings, necklaces, pins, etc. A silverware tray has different slots for forks, knives, spoons.

We can model many such collections using separate variables (if there is only one wallet, jewelry box, silverware tray, ...) or parallel arrays (if we have a collection of such things). But both cases fail to model the notion that the contents of a particular wallet all belong together.

Example: Tracking Course Information

Let's imagine the following situation. You'd like to keep track of your course information. Each course will have the following information associated with it:

• department name
• course number
• number of credits the course is worth
• whether the course is pass/fail or not

Clearly, a homogeneous structure like an array does not suit our needs. However, a struct would be perfect!

struct-demo.cpp.

Miscellaneous struct information

Allocation and initialization.

To create a variable of a structure type (which will allocate enough memory to hold values for all the members), you can do either of the following:

course c1;

struct course c1;

But don't mix the two forms.

You may optionally initialize the members of a structure by giving their values in the order the members were declared:

struct course c1 = {"CS", 11, 3.0, false};

Accessing member variables

You can access the members of struct using dot notation:

• c1.dept refers to the value of the dept member variable within the instance of the course struct named c1.
• The type of c1.dept is string.
• You can get the other elements similarly.

Assignment

• c1.passfail = true; changes the passfail member variable to true.
• c1 = c2; changes all the values of c1 to be equal to all the values of c2. It's as if you assigned each member variable one at a time.

Other information

What can go inside a struct? Anything!

Combining struct with array

• Array of structs. Very useful! A playlist can be an array of songs, where each song has a name, album, artist, play length.
• struct containing an array (WARNING!).

How does a struct differ from a class?

Great question! They are actually quite similar. The key difference is that: in a struct, all members are public by default.

Practice Exercise

You're writing a trip planning application, and you want to include airline flights. Define a struct to model airline flights (what are the elements). Imagine you want to be able to search for a particular flight — what do you need?