What is the main difference between a class and a structure in programming languages?

The main difference between class and struct in programming languages like Swift or C# is that a class is a reference type, while a struct is a value type.

• Class (class): class objects are stored on the heap, and when assigned or passed, a reference to the object is copied. Changes through one reference are reflected in others.
• Struct (struct): when assigned or passed, a copy of the data is created. Changes in the copy do not affect the original.

For example, in Swift:

struct Point {
    var x: Int
    var y: Int
}

class Circle {
    var center: Point
    var radius: Double

    init(center: Point, radius: Double) {
        self.center = center
        self.radius = radius
    }
}

Here, Point is a struct, and copying it creates a new independent instance, while Circle is a class, and its objects are passed by reference.