问题描述
我注意到我们的一些代码中的以下语法有一段时间了:
I noticed for a while now the following syntax in some of our code:
if( NULL == var){
//...
}
或
if( 0 == var){
//...
}
和类似的东西.
谁能解释一下为什么写这篇文章的人选择这个符号而不是常见的var == 0方式)?
Can someone please explain why did the person who wrote this choose this notation instead of the common var == 0 way)?
这是风格问题，还是会以某种方式影响性能?
Is it a matter of style, or does it somehow affect performance?
推荐答案
这是一种避免错误的机制:
It's a mechanism to avoid mistakes like this:
if ( var = NULL ) {
// ...
}
如果你用右边的变量名编写它，编译器将能够发现某些错误:
If you write it with the variable name on the right hand side the compiler will be able catch certain mistakes:
if ( NULL = var ) { // not legal, won't compile
// ...
}
当然，如果变量名出现在等号的两侧并且有些人觉得这种风格不吸引人，那么这当然行不通.
Of course this won't work if variable names appear on both sides of the equal sign and some people find this style unappealing.
正如 Evan 在评论中提到的，如果您启用警告，任何体面的编译器都会警告您这一点，例如，gcc -Wall 会给您以下信息:
As Evan mentioned in the comments, any decent compiler will warn you about this if you enable warnings, for example, gcc -Wall will give you the following:
warning: suggest parentheses around assignment used as truth value
您应该始终在编译器上启用警告，这是查找错误最便宜的方法.
You should always enable warnings on your compiler, it is the cheapest way to find errors.
最后，正如 Mike B 所指出的，这是一个风格问题，不会影响程序的性能.
Lastly, as Mike B points out, this is a matter of style and doesn't affect the performance of the program.
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