第四章语句和声明

<pre name="code" class="sql">$Some::stuff 使用包some的标准变量$stuff


our 把名字限制于一个作用域,

local 把值限制于一个作用域,

my 把名字和值都限制于一个作用域



my变量:
[root@master Webqq]# cat t5.pl 
{my $var=11;print "$var is 1--$var
";}{print "$var is 2--$var
"};
{our $var=11;print "$var is 1--$var
";}{print "$var is 2--$var
"};
[root@master Webqq]# perl t5.pl 
$var is 1--11
$var is 2--
$var is 1--11
$var is 2--11



our变量:

[root@master Webqq]# cat t6.pl 
our $PROGRAM_NAME = "waiter";
{
my $PROGRAM_NAME = "something";
print "$PROGRAM_NAME is 1---$PROGRAM_NAME
";
our $PROGRAM_NAME = "server"; #这里的our和外面的相同，和前句不同。
print "$PROGRAM_NAME is 2----$PROGRAM_NAME
";
# 这里调用的代码看到的是"server"
 }
print "$PROGRAM_NAME is 3----$PROGRAM_NAME
";
[root@master Webqq]# perl t6.pl 
$PROGRAM_NAME is 1---something
$PROGRAM_NAME is 2----server
$PROGRAM_NAME is 3----server
[root@master Webqq]# 


my 声明的是词法变量，"把名字和值都限于限于某个范围",简单说，就是只能本层模块或者函数可以看到这个变量，高一层的或

者低一层的都看不到的。例如：
sub greeting1{
my ($hello) = "How are you do?";
greeting2();
}
sub greeting2{
print "$hello/n";
}
$hello = "How are you doing?";
greeting2();
greeting1();
greeting2();
运行结果：
How are you doing?
How are you doing?
How are you doing?
一个 How are you do? 都没有,在greeting1中call greeting2时，greeting2看不到greeting1的私有 $hello变量，只能看到外

面的全局变量$hello



如果你看到某人写到 $Some::stuff（注：或者 $Some'stuff，不过我们不
鼓励这么写），他们是在使用来自包 Some 的标量变量 $stuff


读取标准输入:

[root@master Webqq]# cat t12.pl 
my $foo = <STDIN>;
print "$foo is $foo
";
[root@master Webqq]# perl t12.pl 
abc123
$foo is abc123



my  ,local ,our 变量:

my 变量:
[root@master Webqq]# cat t12.pl 
my $i=10;
{
my $i=99;
print "$i is 1--$i
";
};
print "$i is 2--$i
";
[root@master Webqq]# perl t12.pl 
$i is 1--99
$i is 2--10


our 全局变量:
[root@master Webqq]# cat t12.pl 
our $i=10;
{
our $i=99;
print "$i is 1--$i
";
};
print "$i is 2--$i
";
[root@master Webqq]# perl t12.pl 
$i is 1--99
$i is 2--99


local 用来临时改变一个现有的全局变量的值

[root@master Webqq]# cat t12.pl 
our $i=10;
{
local  $i=99;
print "$i is 1--$i
";
};
print "$i is 2--$i
";
[root@master Webqq]# perl t12.pl 
$i is 1--99
$i is 2--10


[root@master Webqq]# cat t12.pl 
use strict;
$i=10;
[root@master Webqq]# perl t12.pl 
Global symbol "$i" requires explicit package name at t12.pl line 2.
Execution of t12.pl aborted due to compilation errors.


4.6  全局声明:


库,模块 和类都用包来存储它们的接口数据(以及一些它们的半私有数据)


创建私有变量（my) ,进行有选择访问的全局变量(our)  给全局变量提供临时值的(local)


4.7.3  词法范围全局声明:our

有一个访问全局变量的更好的方法就是our 声明,尤其那些在use strice声明下运行的程序和模块。



zjzc01:/root/big# cat a27.pl 
{$var = aa;print "$var is $var
"};
print "$var is $var
";
zjzc01:/root/big# perl a27.pl 
$var is aa
$var is aa


zjzc01:/root/big# cat a27.pl 
{my $var = aa;print "$var is $var
"};
print "$var is $var
";
zjzc01:/root/big# perl a27.pl 
$var is aa
$var is 


zjzc01:/root/big# cat a27.pl 
{our $var = aa;print "$var is $var
"};
print "$var is $var
";
zjzc01:/root/big# perl a27.pl 
$var is aa
$var is aa


zjzc01:/root/big# cat a27.pl 
{local $var = aa;print "$var is $var
"};
print "$var is $var
";
zjzc01:/root/big# perl a27.pl 
$var is aa
$var is 



4.7.4  动态范围变量:local

zjzc01:/root/big# cat a28.pl 
$var=100;
{local $var=200; print "$var is $var
"};
print "$var is $var
";

zjzc01:/root/big# perl a28.pl 
$var is 200
$var is 100

这也是为什么我们称之为动态范围---因为它是在运行时修改的:

尽管my和local 都提供了某种程度的保护,总的来说你还是应该优先使用my.


当然,有时候你不得不用local来临时改变一个现有全局变量的值。
第四章 语句和声明

第四章语句和声明
