[Perl] useless use of a constant in a void context

最新推荐文章于 2019-06-10 00:10:34 发布

weixin_30379531

最新推荐文章于 2019-06-10 00:10:34 发布

阅读量495

点赞数

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/sanquanfeng/p/3298278.html

本文详细解释了Perl语言中上下文的概念，包括空上下文、标量上下文和列表上下文，并通过具体代码示例说明了如何根据上下文确定表达式的处理方式。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

- Do you understand context? It freaked me out once or fifty times too. =) When perl is compiling your code, it looks at structures like these and decides on what "context" codehere() is in:

codehere(); #void context 
$c=codehere(); #scalar context 
($l)=codehere(); #list context 
@a=codehere(); #list context 
%h=codehere(); #list context 
print codehere(); #list context 
foreach (codehere()) { #list context 
if ( 1 <= codehere() ) { #scalar context

When you do something along the lines of this: $a=( codehere(),codehere() );

then the $a= forces the right side into scalar context since perl can see you don't want a list so in scalar context, the right side isn't treated as a list but as a statement group. Thus, "," is an operator that forces void context on it's left-hand-side and passes on the whatever context it is in to it's right-hand-side. Since you strung together a series of values like this: $a = ( 2, 3, 4, 5, 6 ); the first four numbers are in void context and only the last item in the series is in scalar context. Thus your statement could be rewritten: 2; 3; 4; 5; $a = 6;

And, bam!, from that you can see there are 4 constants in void context. Run either of those with perl -we on the command line and enjoy seeing the 4 warnings pop-up.

BTW, $a = ( 1, 5 ); pulls no error. =) It seems that perl and in fact Perl treat 1; special.

转载于:https://www.cnblogs.com/sanquanfeng/p/3298278.html