Structs

One way to define compound or custom types in D is to use a struct:

struct Person {
    int age;
    int height;
    float ageXHeight;
}

By default structs are constructed on the stack (unless created with new) and are copied by value in assignments or as parameters to function calls.

auto p = Person(30, 180, 3.1415);
auto t = p; // copy

When a new object of a struct type is created, its members can be initialized in the order they are defined in the struct. A custom constructor can be defined through a this(...) member function. If needed to avoid name conflicts, the current instance can be explicitly accessed with this:

struct Person {
    this(int age, int height) {
        this.age = age;
        this.height = height;
        this.ageXHeight = cast(float)age * height;
    }
        ...

Person p = Person(30, 180); // initialization
p = Person(30, 180);  // assignment to new instance

A struct may contain any number of member functions. By default they are public and accessible from the outside. They could also be private and thus only be callable by other member functions of the same struct, or other code in the same module.

struct Person {
    void doStuff() {
        ...
    private void privateStuff() {
        ...

// In another module:
p.doStuff(); // call method doStuff
p.privateStuff(); // forbidden

Const member functions

If a member function is declared with const, it won't be allowed to modify any of its members. This is enforced by the compiler. Making a member function const makes it callable on any const or immutable object, but also guarantees to callers that the member function will never change the state of the object.

Static member functions

If a member function is declared as static, it will be callable without an instantiated object (e.g. Person.myStatic()) but it isn't allowed to access any non-static members. It can be used if a method doesn't need to access any of the object member fields but logically belongs to the same class. Also it can be used to provide some functionality without creating an explicit instance, for example, some Singleton design pattern implementations use static.

Inheritance

Note that a struct can't inherit from another struct. Hierachies of types can only be built using classes, which we will see in a later section. However, with alias this or mixins one can easily achieve polymorphic inheritance.

In-depth

Exercise

Given the struct Vector3, implement the following functions and make the example application run successfully: