定义双重感叹号? [英] Defining double exclamation?

问题描述

我了解双感叹号的作用(或者我想我了解)，但是我不确定如何在随机对象上定义它.例如，在下面的代码段中:

I understand what a double exclamation mark does (or I think I understand) but I am not sure how it is defined on a random object. For example in the code snippet below:

Assignment *a;
if (!getAssignment(query, a))
   return false;
hasSolution = !!a;

if (!a)
   return true;

我怎么知道双感叹号将产生什么值?换句话说，它总是转换为true吗?错误的 ?还是可以为其定义行为，例如执行确定结果的方法(对象如何知道在这种情况下如何行动)?由于发生了所有这些感叹号，我对这段代码有些困惑.

How do I know what value will the double exclamation mark result in ? In other words does it always convert to true ? false ? or can you define a behavior for it such as executing a method to determine the result (how does the object know how to act in this situation) ? I am bit confused about this piece of code due to all these exclamation stuff going on.. Any explanation is appreciated.

希望我很清楚，谢谢.

推荐答案

a是一个指针.在C ++中，nullptr被定义为无效的指针. !pointer将nullptr指针变为true，将非nullptr指针变为false. !boolean将true变成false，将false变成true.它将始终有效.

a is a pointer. In C++, nullptr is defined to be an invalid pointer. !pointer turns a nullptr pointer into true and a non nullptr pointer into false. !boolean turns true into false and false into true. It will always work.

!(!a)是思考它的一种有用方法.

!(!a) is a useful way to think of it.

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