perl - 如何区分函数参数中的变量和文字/常量

Question

在这种情况下，如何识别是否有变量或字面量传递给函数 f()？

如何实现passed_as_constant()检查(见下面代码)？

sub f {
  my $refStr=\$_[0];
  return if passed_as_constant($_[0]) and !defined(wantarray);
  substr(${$refStr}, 0, 1)='D';
# return the copy of the string if it was passed
# to the function as constant aka "literally" or if copying was explicitly requested by the left-side context
  (passed_as_constant($_[0]) or defined(wantarray)) and return ${$refStr};
}
# Argument was passed literally and function called without left-side context - we may simply "do nothing"
f('Bingo!');

# Argument was passed literally and left-side context is here. We must return a copy of literal/constant
say my $s=f('Bingo!');

# Here we can modify the $s "in-place"
f(my $s='Bingo!');

谢谢!

Answer 1

不要这样做。该功能应始终表现相同。就地修改相当不明显。如果要修改值，请明确要求提供引用。这使得修改在调用站点上更加明显。

如果你真的要判断一个参数是否为常数，你可以使用 readonly() from Scalar::Util .在内部，每个标量都有一个标志来控制是否允许修改。这是为文字设置的。对只读标量的分配将失败。使用 readonly() 可以查询这个标志。