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Source Code Fundamentals: Constants

A constant is a named value. Once defined, the value of the constant cannot be changed. A constant can be defined in one of two ways: using the const keyword at the top level, or inside a class or interface. For example:

<?hh // strict

namespace Hack\UserDocumentation\Fundamentals\Constants\Examples\DefiningConstants;

const int MAX_COUNT = 123;
class C {
  const float MAX_HEIGHT = 10.5;
  const float UPPER_LIMIT = C::MAX_HEIGHT;
}

<<__EntryPoint>>
function main(): void {
  echo "    MAX_COUNT = " . MAX_COUNT. "\n";
  echo "   MAX_HEIGHT = " . C::MAX_HEIGHT . "\n";
}
Output 
MAX_COUNT = 123
   MAX_HEIGHT = 10.5

Context-Dependent Constants

The following constants --- sometimes referred to as magic constants --- are automatically available to all scripts; their values are not fixed:

Constant Name Description
__CLASS__ string; The name of the current class. From within a trait method, the name of the class in which that trait is used. If the current namespace is other than the default, the namespace name and "\" are prepended, in that order. If used outside all classes, the value is the empty string.
__DIR__ string; The directory name of the script. A directory separator is only appended for the root directory.
__FILE__ string; The full name of the script.
__FUNCTION__ string; Inside a function, the name of the current function exactly as it was declared, with the following prepended: If a named namespace exists, that namespace name followed by "". If used outside all functions, the result is the empty string. For a method, no parent-class prefix is present. (See __METHOD__ and anonymous functions.)
__LINE__ int; the number of the current source line
__METHOD__ string; Inside a method, the name of the current method exactly as it was declared, with the following prepended, in order: If a named namespace exists, that namespace name followed by ""; the parent class name or trait name followed by ::. If used outside all methods, the result is the same as for __FUNCTION__.
__NAMESPACE__ string; The name of the current namespace exactly as it was declared. For the default namespace, the result is the empty string.
__TRAIT__ string; The name of the current trait. From within a trait method, the name of the current trait. If used outside all traits, the result is the empty string.

Core Predefined Constants

Namespace HH\Lib\Math contains a number of integer-related constants (such as INT64_MAX and INT64_MIN).