if（NULL == pointer）vs if（pointer == NULL）有什么区别？ [英] What is the difference between if (NULL == pointer) vs if (pointer == NULL)?

问题描述

使用之间的区别：

if (NULL == pointer) 

并使用：

if (pointer == NULL)    

我的教授说使用前者而不是后者，两者之间的差异。

My professor says to use the former over the latter but I don't see the difference between the two.

推荐答案

没有区别，它被称为 Yoda条件还可以查看Yoda条件，Pokémon异常处理和其他编程经典。

There is no difference, it is called Yoda conditions also see "Yoda Conditions", "Pokémon Exception Handling" and other programming classics.

应该防止使用赋值 = ）在比较中错误地等于（ == ），但现代编译器现在应该警告这种类型的防御性规划不应该需要。例如：

It is supposed to prevent the usage of assignment(=) by mistake over equality(==) in a comparison, but modern compilers should warn about this now, so this type of defensive programming should not be needed. For example:

if( pointer = NULL )

会将 NULL 分配给指针 ：

if( pointer == NULL )

它应该是一个比较，糟糕。使用 Yoda条件将此错误（ ），并附上类似的讯息：

it should have been a comparison, Oops. Make this an error using Yoda conditions you will (see it live), with a similar message to this:

错误：表达式无法指派

error: expression is not assignable

正如jrok指出的：

if (!pointer)

在这种情况下避免这个问题。

avoids this problem all together in this case.

这里是一个具体的例子，为什么现代编译器我们不再需要这种技术了（ see it live

Here is a concrete example of why with a modern compilers we don't need this technique anymore(see it live):

#include <iostream>

int main()
{
    int *ptr1 = NULL ;

    if( ptr1 = NULL )
    {
            std::cout << "It is NULL" << std::endl ;
    }

}

请注意所有警告： p>

Note all the warnings:

warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
    if( ptr1 = NULL )
        ~~~~~^~~~~~

note: place parentheses around the assignment to silence this warning
    if( ptr1 = NULL )
             ^
        (          )

use '==' to turn this assignment into an equality comparison
    if( ptr1 = NULL )
             ^
             ==

这使得很难错过这个问题。值得注意的是，在 C ++ 中， nullptr 应该优先于 NULL ，您可以查看使用nullptr的好处是什么？所有的细节。

which makes it pretty hard to miss the problem. It is worth noting that in C++ nullptr should be preferred over NULL, you can look at What are the advantages of using nullptr? for all the details.

请注意，在 C ++

请注意， - Wparentheses警告在某些方面强制样式选择，您需要放弃潜在的有效使用分配产生警告的位置，例如，如果您使用 -Werror 或选择括号括起这些情况，有些人可能会发现丑陋建议。我们可以使用 -Wno-parentheses gcc 和 clang c $ c>但我不会推荐这个选择，因为警告一般会指示一个真正的错误。

Note, the -Wparentheses warning in some ways forces a style choice, you either need to give up potentially valid uses of assignment in places where the warning is generated, for example if you use -Werror or choose to parenthesize those cases, which some may find ugly as the comment below suggests. We can turn of the warning in gcc and clang using -Wno-parentheses but I would not recommend that choice since the warning in general will indicate a real bug.

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