Why can't a default constructor have parameters in EK9?

← Classes and OOP · Ref: Q776

The 'default' keyword on a constructor means 'compiler, generate a zero-argument constructor that initialises all fields to their declared values'. Adding parameters contradicts this — the compiler cannot know how to map parameters to fields.

DEFAULT CONSTRUCTOR RULES

- 'default ClassName()' — no parameters, compiler generates body
- All fields must be initialised at declaration for default to work
- You can have BOTH a default constructor AND parameterised constructors

THIS EXAMPLE

The Config class has two fields initialised at declaration and uses 'default Config()' for zero-argument construction. It also has a parameterised constructor for custom values. Both coexist.

WHEN TO USE DEFAULT

Use default when all fields have sensible initial values and you want a zero-argument constructor without writing the body. Use a regular constructor when you need parameters.

See Q89 for constructor basics. See Q82 for default operators. See Q774 for abstract constructor rules.

Example

defines module qa.classesandoop.defaultconstructor

  defines class

    Config
      name <- "default"
      timeout <- 30

      default Config()

      Config()
        ->
          n as String
          t as Integer
        name :=: n
        timeout :=: t

      describe()
        <- rtn as String: `${name} timeout=${timeout}`

      default operator ?

    Setting
      label <- "none"
      enabled <- false

      default private Setting()

      Setting()
        -> l as String
        label :=: l
        enabled: true

      default operator ?

  defines program

    ShowConfig()
      stdout <- Stdout()
      defaults <- Config()
      custom <- Config("prod", 60)
      if defaults?
        stdout.println(defaults.describe())
      if custom?
        stdout.println(custom.describe())

Common mistakes

E07080 — Adding a parameter to a default constructor is invalid. The 'default' keyword means zero-argument auto-generated constructor. Either remove 'default' and write a parameterised constructor, or remove the parameter. See ek9 -h E07080 for details.

Incorrect:

      default Config()
        -> name as String

Correct:

      default Config()

E07080 — Even with the private modifier, a default constructor cannot have parameters. Default means zero-arg. Remove 'default' to create a parameterised private constructor. See ek9 -h E07080 for details.

Incorrect:

      default private Setting()
        -> initial as Integer

Correct:

      default private Setting()

Coming from another language?

Java: default constructor is implicit (no-arg, compiler-generated if no constructors defined). Python: __init__ with defaults provides similar pattern. Rust: Default trait derive provides zero-arg construction. Kotlin: default parameter values on primary constructor. Go: zero-value initialization is automatic. EK9: explicit 'default ClassName()' keyword, must be zero-arg, fields must be initialised.

Keywords: E07080, initialise, parameters, default, zero-arg, generate, constructor, class, fields