Structs (struct vs class)

Real data comes in clusters: a point has an x and a y, a student has a name and a score, an order has an item, quantity, and price. A struct lets you bundle those related values into one named type so you can pass them around as a single thing. By the end of this lesson you'll define structs, store them in vectors, give them member functions, and understand exactly how struct and class differ.

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A struct is a form with labelled fields . A "Contact" form has lines for Name, Phone, and Email — three different bits of information that belong together on one sheet. You don't carry around three loose scraps of paper; you keep one form. In code, a Contact struct holds those fields together, so you can pass one Contact to a function instead of three separate arguments. A class is the same form, but locked in a drawer where you must go through a clerk (public methods) to read or change anything — that's encapsulation.

1. Defining and Using a Struct

You declare a struct with the struct keyword, a name, and a list of members (the fields). Create an instance, then read and write its members with the dot operator . . The tidiest way to set everything at once is aggregate initialization with braces, which fills the members in declaration order. Don't forget the semicolon after the closing brace of the struct definition.

Your turn. Fill in the brace initializer to create a Book with the members in order:

These lines define a Point struct and use it. Put them in the correct order:

Define the Point type first, before main (A), open main (C), create a point (D), print x + y = 7 (B), then return 0; (E) and close the brace (F). The struct must be declared before it's used.

2. struct vs class

Here's the question every C++ beginner asks. The only language-level difference is the default access level : in a struct , members are public by default; in a class , they're private by default. Everything else — constructors, methods, inheritance — is identical. By convention, programmers use struct for simple bundles of mostly-public data and class for types that hide their internals behind public methods (encapsulation).

3. Structs With Behaviour

A struct isn't limited to data — it can carry constructors (to set up its members) and member functions (to compute things from them). A member function marked const promises not to modify the struct, which both documents intent and lets you call it on const objects. This is exactly where a struct starts to look like a class.

4. A Vector of Structs

The single most useful pattern in everyday C++ is a — a table of records. Each element is one row (a student, an order, a point), and you can loop, filter, and sort them with a lambda comparator. This combines structs with the vectors, range-based loops, and lambdas you've already learned.

You have several ways to fill a struct, each with its place:

When passing a struct to a function, remember the cost of copying. Pass by const& to read large structs without a copy, and by & when the function must modify the original:

Predict the output before revealing the answer.

10 — brace init sets x = 2, y = 5 in order, so 2 * 5 = 10.

No — in a class , v is private by default, so b.v is inaccessible. A struct (public by default) would compile.

6 — area() returns w * h = 2.0 * 3.0 = 6. Structs can have member functions.

Design your own Item struct, store several in a vector, and total their value. This is the exact pattern behind carts, ledgers, and dashboards.

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