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pop_backx

Note

This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information.

Removes the last element from a Container and returns it

namespace HH\Lib\C;

function pop_backx<T as Container<Tv>, Tv>(
  inout T $container,
): Tv;

If the Container is empty, an InvariantException is thrown.

When an immutable Hack Collection is passed, the result will be defined by your version of hhvm and not give the expected results.

For maybe empty Containers, see pop_back. To get the first element, see pop_frontx.

Time complexity: O(1 or N) If the operation can happen in-place, O(1) if it must copy the Container, O(N). Space complexity: O(1 or N) If the operation can happen in-place, O(1) if it must copy the Container, O(N).

Parameters​

  • inout T $container

Returns​

  • Tv

Examples​

$strings = vec["a", "b"];
$pop_backx_result_1 = C\pop_backx(inout $strings);
echo "First pop_backx result: $pop_backx_result_1\n";
//Output: First pop_backx result: b

$empty_strings = vec[];
$pop_backx_result_2 = C\pop_backx(inout $empty_strings);
//Output: Hit a php exception : exception 'InvariantViolationException' with message 
//'HH\Lib\C\pop_backx: Expected at least one element'