SplDoublyLinkedList::shift()

(PHP 5 >= 5.3.0, PHP 7)

Shifts a node from the beginning of the doubly linked list

说明

publicSplDoublyLinkedList::shift(void) : mixed

参数

此函数没有参数。

返回值

The value of the shifted node.

错误／异常

ThrowsRuntimeExceptionwhen the data-structure is empty.

One may expect SplDoublyLinkedList::shift to properly maintain internal pointers, but this is not the case, this will yield no results, even if you rewind first
<?php
while ($splDoublyLinkedList->valid()) {
  yield $splDoublyLinkedList->shift();
}
?>
It could be by design, but the following raises some more questions :
<?php
$test = new \SplDoublyLinkedList;
$dataSet = [
  ['id' => 1],
  ['id' => 2],
  ['id' => 3],
  ['id' => 4],
];
foreach ($dataSet as $row) {
  $test->push($row);
}
echo "count: " . $test->count() . PHP_EOL;
echo "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
echo "current: " . var_export($test->current(), true) . PHP_EOL;
echo "key: " . $test->key() . PHP_EOL;
echo "1st shift: " . var_export($test->shift(), true) . PHP_EOL;
echo "count: " . $test->count() . PHP_EOL;
echo "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
echo "current: " . var_export($test->current(), true) . PHP_EOL;
echo "key: " . $test->key() . PHP_EOL;
echo "2nd shift: " . var_export($test->shift(), true) . PHP_EOL;
echo "count: " . $test->count() . PHP_EOL;
echo "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
echo "current: " . var_export($test->current(), true) . PHP_EOL;
echo "key: " . $test->key() . PHP_EOL;
echo "rewinding... " . PHP_EOL;
$test->rewind();
echo "current: " . var_export($test->current(), true) . PHP_EOL;
echo "2nd shift: " . var_export($test->shift(), true) . PHP_EOL;
echo "count: " . $test->count() . PHP_EOL;
echo "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
echo "current: " . var_export($test->current(), true) . PHP_EOL;
echo "key: " . $test->key() . PHP_EOL;
?>
will result in :
<?php
/*
count: 4        
valid: false            <== First thing to note, you have no valid pointer unless you rewind first
current: NULL         <== hence no current
key: 0                <== but we have a valid key
offsetGet(key): array (  <== indeed
 'id' => 1,      
)            
1st shift: array (        <== and shift does return first row
 'id' => 1,      
)            
count: 3              <== and count is maintained as expected
valid: false           <== but internal pointer was left outside valid range
current: NULL         <== hence no current again
key: 0                <== but we still have a valid key
offsetGet(key): array (  <== indeed
 'id' => 2,      
)            
2nd shift: array (       <== and shift does return first row
 'id' => 2,      
)            
count: 2              <== and count is maintained as expected
valid: false           <== still not valid etc ..
current: NULL      
key: 0         
offsetGet(key): array ( 
 'id' => 3,      
)            
rewinding...          <== now rewind
current: array (       <== yay a current
 'id' => 3,      
)            
3rd shift: array (      <== shift ok
 'id' => 3,      
)            
count: 1             <== count ok
valid: true           <== ouch valid 
current: NULL        <== with no current
key: 0               <== still our key is valid :o
offsetGet(key): array ( 
 'id' => 4,      
)              
*/
?>
Conclusion : I may be missing something about why SplDoublyLinkedList::shift is not maintaining proper internal index in the first place, but I find it even more confusing to be able to end up with a valid valid() and a valid key() and no current() while there is obviously one.
Tested on php 5.6.30 & 7.1.2 with the exact same result.