$x = undef和undef $x之间的行为差异

Question

undef $x is different from $x = undef。我的印象是两者都会触发垃圾回收和释放内存，但$x = undef似乎没有这样做。

这是一个语言错误吗？在做$x = undef的时候，它不应该释放内存吗？

Answer 1

不是也不是。与内存使用相比，Perl更看重速度，因为它不会释放您可能再次需要的内存。如果您希望释放字符串缓冲区，请使用undef $x;。

$ perl -MDevel::Peek -e'
   Dump($x);
   $x='abc'; Dump($x);
   $x=undef; Dump($x);
   undef $x; Dump($x);
'
SV = NULL(0x0) at 0x1c39284       <-- No body allocated
  REFCNT = 1
  FLAGS = ()                      <-- Undefined
SV = PV(0x3e8d54) at 0x1c39284    <-- PV body allocated
  REFCNT = 1
  FLAGS = (POK,pPOK)              <-- Contains a string
  PV = 0x3eae7c "abc"\0
  CUR = 3
  LEN = 12
SV = PV(0x3e8d54) at 0x1c39284    <-- PV body allocated
  REFCNT = 1
  FLAGS = ()                      <-- Undefined
  PV = 0x3eae7c "abc"\0           <-- Currently unused string buffer
  CUR = 3
  LEN = 12
SV = PV(0x3e8d54) at 0x1c39284    <-- PV body allocated
  REFCNT = 1
  FLAGS = ()                      <-- Undefined
  PV = 0                          <-- No string buffer allocated