Built In Types: Classname

For the most part, we deal with class types directly via their names. For example:

class Employee {
  // ...
}

$emp = new Employee();

However, in some applications, it is useful to be able to abstract a class' name rather than to hard-code it. Consider the following:

<<__ConsistentConstruct>>
class Employee {
  // ...
}

function f(classname<Employee> $clsname): void {
  $w = new $clsname();  // create an object whose type is passed in
}

This function can be called with the name of the class Employee or any of its subclasses.

class Intern extends Employee {
  // ...
}

function demo(): void {
  f(Employee::class);  // will call: new Employee();
  f(Intern::class);    // will call: new Intern();
  f(Vector::class);    // typechecker error!
}

In Hack code, the class names must be specified using "::class literals" (SomeClassName::class). At runtime, these are regular strings (SomeClassName::class === 'SomeClassName').

The value of an expression of a classname type can be converted implicitly or explicitly to string.