它是可以接受的'NULL'分配给布尔数据类型？ [英] Is it acceptable to assign 'NULL' to a boolean datatype?

问题描述

它是可以接受的'NULL'分配给布尔数据类型？

Is it acceptable to assign 'NULL' to a boolean datatype?

推荐答案

从理论的角度来看，是的。但它做了可怕的事情。

From a theoretical point of view, yes. But it's a horrible thing to do.

NULL 是分配给一个指针，它指向任何一个空指针常量。

NULL is a null pointer constant that is assigned to a pointer to make it point to nothing.

...
ptr = NULL; // now it points to no object anymore
...

// or ptr == 0
if(ptr == NULL) {
    ...
}

---

---

下面是标准的参考，如果你有兴趣在任何情况下。首先，空指针常数（ 4.10 / 1 ）

Here are the references to the Standard if you are interested in any case. First, a null pointer constant is (4.10/1)

一个空指针常量是一个整型常量前pression计算结果为零整数类型（5.19）右值。

A null pointer constant is an integral constant expression (5.19) rvalue of integer type that evaluates to zero.

那么，如果我们把一个空指针常量布尔会发生什么？它在解释 4.12 / 1 ：

Then, what happens if we convert a null pointer constant to bool? It's explained in 4.12/1:

算术，枚举，指针，或指针到成员类型右边的值可以转换为bool类型的右值。零值，空指针值或空成员指针值转换为false;一个

An rvalue of arithmetic, enumeration, pointer, or pointer to member type can be converted to an rvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; a

（当谈到右值这基本上意味着相对于该类型的变量一个简单的值）。

(When it talks about rvalue it essentially means a simple value as opposed to a variable of that type).

现在，什么是真正的是的是 NULL ？阅读 18.1 / 4 （请注意，在C，一个空指针常量定义不同。这可能是它明确提到C ++的原因）

Now, what actually is that NULL? Read 18.1/4 (note that in C, a null pointer constant is defined differently. Which might be the reason it explicitly refers to C++)

宏NULL是本国际标准实现定义的C ++空指针常量

The macro NULL is an implementation-defined C++ null pointer constant in this International Standard

这是组合的最重要的一点是部分零值...被转换为false 。 NULL 的一个bool变量的赋值将尝试 NULL 转换为布尔值。作为上述段落说，这样的转换存在，并且可以做到的。

The important bit of that combination is the part "A zero value ... is converted to false". An assignment of NULL to a bool variable will try to convert NULL to a boolean value. As the above passages say, such a conversion exist and can be done.

有关空指针了解另一个重要的事情是空指针和之间的不同的空指针常量。正如我们刚才读，空指针常量是一些整数值，这就是零。然而，空指针和它的值，空指针值，是指针和它们的类型是指针类型。下面有int类型，是一个空指针常量

Another important thing about null pointers to understand is the different between a null pointer and a null pointer constant. As we just read, a null pointer constant is some integer value that's zero. However, a null pointer and its value, a null pointer value, are pointers and their type is of pointer type. The following has type int and is a null pointer constant

#define NULL ('n'-'n') // looks suspicious, but conforms

由于它是一个整型常量前pression（即本质上是在编译时已知的整数值）与零值。下面是一个空指针值

Because it is a integral constant expression (that is essentially an integer value that is known at compile time) with value zero. The following is a null pointer value

(void*)NULL

但它的不的空指针常量。但无论如何，也空指针的值的被转换为bool作为上述报价告诉： A ..空指针值..被转换为false 。所以，你无一不精。

but it is not a null pointer constant. But anyway, also null pointer values are converted to bool as the above quote tells: "A .. null pointer value .. is converted to false". So you are all fine.

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