Classes: Type Constants

Type constants provide a way to abstract a type name. However, type constants only make sense in the context of interfaces and inheritance hierarchies, so they are discussed under those topics.

For now, the declaration of a type constant involves the keywords const type. Without explanation, here's an example:

<?hh // strict

namespace Hack\UserDocumentation\Classes\TypeConstants\Examples\Simple;

abstract class CBase {
  abstract const type T;
  // ...

class CString extends CBase {
  const type T = string;
  // ...

A type constant has public visibility and is implicitly static.

By convention, type constant names begin with an uppercase T.

See $$ and $$ for more information.