“if"中的变量赋值状况 [英] Variable assignment in "if" condition

问题描述

我最近浪费了一些时间来找出我的代码中的错误，该错误是由打字错误引起的:

I recently just lost some time figuring out a bug in my code which was caused by a typo:

if (a=b)

代替:

if (a==b)

我想知道是否有任何特殊情况您希望在 if 语句中为变量赋值，或者如果没有，为什么编译器不抛出警告或错误?

I was wondering if there is any particular case you would want to assign a value to a variable in a if statement, or if not, why doesn't the compiler throw a warning or an error?

推荐答案

if (Derived* derived = dynamic_cast<Derived*>(base)) {
   // do stuff with `derived`
}

尽管这经常被引用为反模式(使用虚拟调度！")，但有时 Derived 类型具有 Base 根本没有的功能(并且，因此，不同的功能)，这是打开这种语义差异的好方法.

Though this is oft cited as an anti-pattern ("use virtual dispatch!"), sometimes the Derived type has functionality that the Base simply does not (and, consequently, distinct functions), and this is a good way to switch on that semantic difference.

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