Local and Global variables in perl

问题:

I am having few doubts for local/our scope in perl. I read lot of documentation but still confusion is there. Following are the confusions

1. What is local scope?

   what I read is -> local copies the value of global variable, change the value, user will use it and outside the block it will retain the global value

   Confusion -> my does the same thing. Only benefit I see is that some variables like $package::var cannot be declared with my scope but can be declared with local scope. What else for local

2. What is "global" variable?

What is read is -> Its scope is within the package. Basically we put the global variable in @EXPORT array and use it or append the namespace with it to use in other packages.

doubt -> Again if we declare variable with my scope in main only then we can access the variable throughout the package. Is that right? Is it possible to add the my scoped variables in @EXPORT array and use it in another packages?

I think global variables are declared with our keyword. Is there any other way to do so?

This question may look like repetitive but i am confused

回答1:

In terms of scoping, there are two kinds of variables in Perl.

• Lexical variables are lexically scoped, which means they are only visible in the current lexical scope.
• Package variables are globally scoped, which means they are visible by all code in the interpreter.

Here are ways to create variable.

• my creates a lexical variable.
• our creates a lexical variable that is aliased to the variable of the same name in the current package. In other words, our $foo; is the same as alias my $foo = $The::Current::Package::foo;.
• Global variables are created on use.

local doesn't create any variables. It simply backs up a variable until the current lexical scope is destroyed.

---

my does the same thing.

No. local does not change the scope of a variable. While a lexical variable is only visible in a lexical scope, a localized global variable is still visible across the entire interpreter.

$x = 123;
sub foo { print "$x\n"; }
{ local $x = 456; foo(); }  # 456
foo();                      # 123

$x = 123;
sub foo { print "$x\n"; }
{ my $x = 456; foo(); }   # 123
foo();                    # 123

---

What else for local

local is primarily used to approximate the functionality of my for variables that cannot otherwise be declared lexically.

(Historically, that was all variables. Since 5.6, only punctuation variables cannot be declared lexically.)

---

What is "global" variable?

A variable that can seen globally, i.e. by any code in the interpreter.

---

Is it possible to add the my scoped variables in @EXPORT array and use it in another packages?

No. @EXPORT is used by Exporter. Exporter would not be able to find anything but global symbols (since files are compiled in fresh lexical scopes), so @EXPORT must only contain global symbols.

回答2:

There are two kinds of variables, lexically scoped and globally scoped.

In Perl before version 5, there was only globally scoped. These variables are the package variables. These variables are available everywhere in the program if you use the package prefix.

The local keyword was introduced to provide a way to alter the value of one of these package global variables inside a limited scope, such as inside one subroutine. It will save the old value on a stack when entering the scope with the local statement, and upon exiting, it will restore the old value. These are still package globals, which means that they are still available everywhere. If you are inside a scope with a local variable, and you call a subroutine, that variable is still visible inside that subroutine.

The my keyword was introduced in version 5, and provides lexically scoped variables. These variables only exist inside the scope where they are declared. This means that if you call a subroutine, that my variable is not visible. Upon exiting a scope, the my variables simply go away. You should prefer to use my variables when possible, because you do not want your variables to be visible inside subroutines that you call. You cannot use these type of variables in the @EXPORT list because these variables are not visible outside of their scope.

Finally, the our keyword is a combination of both, in that it gives you a variable that is a package global, but that variable is lexically scoped. This means it will be available anywhere in the program, but at the end of the enclosing block, you cannot refer to that variable any more.

回答3:

Example 1:

sub mess_with_foo {
      $foo=0;
 }

 sub myfunc {
      my $foo=20;
      mess_with_foo();
      print $foo;
 }
 myfunc();

Example 2:

 sub mess_with_foo {
      $foo=0;
 }

 sub myfunc {
      local $foo=20;
      mess_with_foo();
      print $foo;
 }
 myfunc();

Example 1 prints 20 because mess_with_foo() could not see my $foo. It could not change it. my $foo can only be seen in its scope of myfunc().

Example 2 prints 0 because mess_with_foo() can see my $foo and change it. local $foo can be seen in its scope of myfunc() AND in the scope of any function called from within its scope of myfunc().

That's the only difference. Neither my $foo nor local $foo will be seen outside of their scope of myfunc().