Sentinel¶
A sentinel is a special, reserved value used to signal a specific condition, such as the end of a sequence, an absence of data, or an error, instead of adding a separate flag or an exception for it. A sentinel is drawn from the very same domain as legitimate data, which creates ambiguity whenever it collides with an actual value: this is exactly the mechanism behind the famous strpos() syndrome.
Common sentinels include -1 as a ‘not found’ index, the null terminator \0 marking the end of a C string, EOF while reading a file, and a dedicated, otherwise-unreachable object used as the default value of a parameter, to detect that no argument was actually passed.
PHP relies on sentinels extensively: false is the sentinel returned by many string and array functions to mean ‘nothing found’, feof() is the sentinel condition of a read loop, and INF, -INF and NAN act as sentinels for numeric overflow and invalid operations.
<?php
$handle = fopen('data.txt', 'r');
while (!feof($handle)) { // feof() is the sentinel condition
$line = fgets($handle);
}
// a dedicated object as a sentinel default, to detect a missing argument
final class Undefined {}
const UNDEFINED = new Undefined();
function greet(string $name, mixed $mood = UNDEFINED) {
if ($mood === UNDEFINED) {
$mood = 'happy';
}
}
?>
See also Sentinel value.
Related : Strpos() Syndrome, Null, False, Default Value, , array_search
