What's the difference between “local” and “my” in Perl

概述There are two kinds of variable scopes in Perl: Global variables: They reside in the current package, can be accessed from the outside and can have "local" values. The name can be used as a key in the

There are two kinds of variable scopes in Perl:

Global variables: They resIDe in the current package,can be accessed from the outsIDe and can have "local" values. The name can be used as a key in the "stash",the package variable hash / the symbol table. Lexical variables: They resIDe in the current scope (roughly delimited by curly braces). There is no symbol table that can be inspected.

Lexical variables and global variables do not interfere,there can be two different variables with the same name.

Most Perl variable magic happens with global variables: These lines are rougly equivalent:

our $var; $::var; $main::var; ${var}; ${'var'}; local $var; 

but not my $var.

So we can write:

@::array = qw(a b c); my @secondarray = @{array}; 

Which copIEs the arrays. We can also look up the array with a name that is stored in a variable:

my $name = "array"; @secondarray = @{$name}; 

The last line abbreviates to … = @$name.

This is not possible with lexical vars because they do not resIDe in the stash.

The local function assigns a "local" value to a global variable (and globals only) within the current scope and in the scope of all subs that are called from within this scope.

Originally (in Perl 4) meddling with variable names and the stash was the only way to simulate references. These usages are Now mostly outdated by ~2 decades as references are available (what is far safer)

There is a perldoc entry which answers this question in perlfaq7:

What's the difference between dynamic and lexical (static) scoping? Between local()and my()?

local($x) saves away the old value of the global variable $x and assigns a new value for the duration of the subroutine which is visible in other functions called from that subroutine. This is done at run-time,so is called dynamic scoping. local() always affects global variables,also called package variables or dynamic variables. my($x) creates a new variable that is only visible in the current subroutine. This is done at compile-time,so it is called lexical or static scoping. my() always affects private variables,also called lexical variables or (improperly) static(ly scoped) variables.

For instance:

sub visible {     print "var has value $var\n"; } sub dynamic {     local $var = 'local';   # new temporary value for the still-global     visible();              #   variable called $var } sub lexical {     my $var = 'private';    # new private variable,$var     visible();              # (invisible outsIDe of sub scope) } $var = 'global'; visible();              # prints global dynamic();              # prints local lexical();              # prints global 

Notice how at no point does the value "private" get printed. That's because $var only has that value within the block of the lexical() function,and it is hIDden from the called subroutine.

In summary, local() doesn't make what you think of as private,local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables.

See Private Variables via my() in perlsub and Temporary Values via local() in perlsub for excruciating details.

. 9/2/2012 Update  This weekend,I reread the book of Effective Programming Perl,and found more full answer in the section: Item 43 Understand the difference between my and local.  

Reference: http://stackoverflow.com/questions/12189865/whats-the-difference-between-local-and-my-in-perl#

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