对$$如何理解

bandaotidejia

$是声明一个变量

$$是变量的变量吗？

它和c语言的* 指针 **指针的指针是不是一个意思？

minirain

bandaotidejia 发表于 2015-02-01 13:31
$是声明一个变量

$$是变量的变量吗？

用命令：perldoc perlvar 可以查寻得到帮助如下：

1. \$\$      The process number of the Perl running this script. Though you
   
2.             *can* set this variable, doing so is generally discouraged,
   
3.             although it can be invaluable for some testing purposes. It will
   
4.             be reset automatically across "fork()" calls.
   
5. 
   
6.             Note for Linux and Debian GNU/kFreeBSD users: Before Perl
   
7.             v5.16.0 perl would emulate POSIX semantics on Linux systems
   
8.             using LinuxThreads, a partial implementation of POSIX Threads
   
9.             that has since been superseded by the Native POSIX Thread
   
10.             Library (NPTL).
    
11. 
    
12.             LinuxThreads is now obsolete on Linux, and caching "getpid()"
    
13.             like this made embedding perl unnecessarily complex (since you'd
    
14.             have to manually update the value of $), so now $ and
    
15.             "getppid()" will always return the same values as the underlying
    
16.             C library.
    
17. 
    
18.             Debian GNU/kFreeBSD systems also used LinuxThreads up until and
    
19.             including the 6.0 release, but after that moved to FreeBSD
    
20.             thread semantics, which are POSIX-like.
    
21. 
    
22.             To see if your system is affected by this discrepancy check if
    
23.             "getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false
    
24.             value. NTPL threads preserve the POSIX semantics.
    
25. 
    
26.             Mnemonic: same as shells.

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bandaotidejia

这不是我想要的答案 比如
  $abc= {};
  $$abc{"par1"} = {};
  $$abc{"par2"}     = {};
  $$abc{"par3"}       = {};

很明显就是二维的概念

jason680

回复 3# bandaotidejia

Here you are


$ perldoc perldata
NAME
    perldata - Perl data types
...


$ perldoc perlref
NAME
    perlref - Perl references and nested data structures
...

bandaotidejia

所以说，2楼说的不对

jason680

回复 5# bandaotidejia

>>所以说，2楼说的不对

也有可能是,你问的不对...
$$本身就是perl的特殊变量

$ perldoc perlvar
NAME
    perlvar - Perl predefined variables
...

    $PROCESS_ID
    $PID
    $$      The process number of the Perl running this script. You should
            consider this variable read-only, although it will be altered
            across "fork()" calls.

            Note for Linux users: on Linux, the C functions "getpid()" and
            "getppid()" return different values from different threads. In
            order to be portable, this behavior is not reflected by $$, whose
            value remains consistent across threads. If you want to call the
            underlying "getpid()", you may use the CPAN module "Linux::Pid".

            Mnemonic: same as shells.

   

bandaotidejia

我说的很明白了 $是。。。 $$是什么

我没问$$本身

但凡学过perl的 应该知道哪个更不好理解

Cyg07

$本身是一个scalar，$$就是解引用获得引用的scalar。

flw

回复 5# bandaotidejia


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人家辛辛苦苦贴那么多又是找材料又是排版，回头你说人家不对？
读书读傻了吧你？

minirain

flw 发表于 2015-02-04 09:23
回复 5# bandaotidejia

支持版主，有些人提问有问题，还自以为是！
解气！！！
本来$$与$$abc是两个不同范畴：
1、$$是特殊变量；
2、$$abc看过引用人都知道$$abc是解引用，知道$abc是标量引用或者提问者的$abc{'xyz'}为用哈希储存标量引用，如：$abc{'xyz'} ＝\$X;
那么$$abc{'xyz'｝＝$X。