while(foo) vs while(foo != NULL) [英] while(foo) vs while(foo != NULL)

问题描述

有人能解释一下 while(foo) 和 while(foo != NULL) 是如何等效的吗?还有:

Can someone explain how while(foo) vs while(foo != NULL) are equivalent? Also:

while(!foo) vs while(foo == NULL)

我知道！不是，但这就是我所知道的.

I know that ! is not, but that is about all I know.

推荐答案

假设 foo 是指针类型，while (foo) 和 while (foo != NULL) 完全等效.两者也等价于 while (foo != 0).(在我看来 while (foo != NULL) 更清楚地表达了意图.)

Assuming foo is of pointer type, while (foo) and while (foo != NULL) are exactly equivalent. Both are also equivalent to while (foo != 0). (In my opinion while (foo != NULL) more clearly expresses the intent.)

在任何需要条件的上下文中(if()、while()、其他一些)，表达式都被视为 true如果条件比较不等于零，假如果比较等于零.

In any context requiring a condition (if(), while(), a few others), the expression is treated as true if the condition compares unequal to zero, false if it compares equal to zero.

NULL 是一个宏，它扩展为实现定义的空指针常量.将指针值与 NULL 进行比较，将为空指针生​​成真值，为任何非空指针生成假值.

NULL is a macro that expands to an implementation-defined null pointer constant. Comparing a pointer value to NULL yields a true value for a null pointer, a false value for any non-null pointer.

常量0是一个空指针常量[*].(这并不意味着空指针具有 0x00000000 或类似的相同位表示，尽管它经常这样做；这意味着常量 0 in源代码 可以表示空指针.)正如您所料，将指针值与空指针常量进行比较会告诉您该指针是否为空指针.在 while (foo) 中，与零的比较是隐式的——但它仍然测试 foo 是否为空指针.

The constant 0 is a null pointer constant [*]. (This doesn't mean that a null pointer has the same bit representation of 0x00000000 or something similar, though it often does; it means that a constant 0 in source code can denote a null pointer.) As you'd expect, comparing a pointer value to a null pointer constant tells you whether that pointer is a null pointer or not. In while (foo), the comparison to zero is implicit -- but it still tests whether foo is a null pointer or not.

更一般地，while (foo) 将 foo 与 0 进行比较，相当于将其与适当类型的零"进行比较.while (foo) 总是等价于 while (foo != 0).对于浮点值，它也等价于 while (foo != 0.0).对于字符值，它等价于 while (foo != '\0').而且，正如我们所见，对于一个指针值，它等价于 while (foo != NULL).

More generally, while (foo) compares foo to 0, which is equivalent to comparing it to "zero" of the appropriate type. while (foo) is always equivalent to while (foo != 0). For a floating-point value, it's also equivalent to while (foo != 0.0). For a character value, it's equivalent to while (foo != '\0'). And, as we've seen, for a pointer value, it's equivalent to while (foo != NULL).

(在 C 中，如果条件为假，比较运算符总是产生 0 的 int 值，如果条件为真则产生 1 --但任何非零值都被视为真，通过与零的隐式不等式比较.)

(In C, a comparison operator always yields an int value of 0 if the condition is false, 1 if it's true -- but any non-zero value is treated as true, via the implicit inequality comparison to zero.)

[*] 空指针常量被定义为一个值为 0 的整数常量表达式，或者这样的表达式转换为 void*>.空指针常量不一定是指针类型，但转换它为指针类型会产生一个空指针值.将指针值与 0 进行比较会导致 0 隐式转换为指针类型，以便进行比较.

[*] A null pointer constant is defined as an integer constant expression with the value 0, or such an expression cast to void*. A null pointer constant isn't necessarily of pointer type, but converting it to pointer type yields a null pointer value. Comparing a pointer value to 0 causes the 0 to be implicitly converted to the pointer type so the comparison can be done.

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