为什么在比较变量之前将恒量？ [英] Why put the constant before the variable in a comparison?

问题描述

我注意到了一段时间，现在在我们的一些code的语法如下：

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。

有人能解释一下为什么当初是谁写的这个选择这种符号，而不是常见的 VAR == 0 办法的人）？

Can someone please explain why did the person who wrote this choose this notation instead of the common var == 0 way)?

它是一个风格问题，还是它在某种程度上影响性能？

Is it a matter of style, or does it somehow affect performance?

推荐答案

这是为了避免这样的错误的机制：

It's a mechanism to avoid mistakes like this:

if ( var = NULL ) {
  // ...
}

如果您对右手边的变量名写出来，编译器将能够捕获某些错误：

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，如果变量名称出现在等号的两侧，有些人觉得这种风格吸引力，这将不起作用。

Of course this won't work if variable names appear on both sides of the equal sign and some people find this style unappealing.

编辑：

由于埃文在评论中提到，任何像样的编译器会发出警告，如果启用了警告，例如的gcc -Wall 会给你以下内容：

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

您应该总是在你的编译器支持的警告，这是为了发现错误最便宜的方式。

You should always enable warnings on your compiler, it is the cheapest way to find errors.

最后，麦克乙组分出来，这是一个风格问题，并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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