在“if”中的变量赋值条件 [英] Variable assignment in "if" condition

问题描述

我最近刚忘了在我的代码中导致一个错字造成的错误：

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

if(a=b)

而不是：

if(a==b)

是任何特殊情况下，你会想要分配一个变量在 if 语句中，或者如果不是，为什么编译器不会引发警告或错误？ / p>

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`
}

引用为反模式（使用虚拟分派！），有时 Derived 类型具有 Base 根本不是（因此，不同的功能），这是一个好的方式来打开那个语义差异。

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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