相比之下，我可以假设 NULL 值为假吗？

Question

我知道 NULL == (void *)0 但有人提到它可以表示为不包含全零的值。令我困扰的是，如果这些代码片段对于所有 (any_type *) 都是等效的:

any_type *val;

if (val) { ... };

和

if (val != NULL) { ... };

Answer 1

是的。您可以通过这种方式使用宏 NULL。它是一个等于0 的空指针常量。

comp.lang.c FAQ list · Question 5.3 :

When C requires the Boolean value of an expression, a false value is inferred when the expression compares equal to zero, and a true value otherwise. That is, whenever one writes

if(expr)

where expr is any expression at all, the compiler essentially acts as if it had been written as

if((expr) != 0)

Substituting the trivial pointer expression p for expr, we have

if(p)   is equivalent to    if(p != 0)

and this is a comparison context, so the compiler can tell that the (implicit) 0 is actually a null pointer constant, and use the correct null pointer value. There is no trickery involved here; compilers do work this way, and generate identical code for both constructs. The internal representation of a null pointer does not matter.

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but it is mentioned that it can be represented as a value which doesn't contain all zeros.

是的。真的。但是representation doesn't matter在这种情况下:

Whenever a programmer requests a null pointer, either by writing 0 or NULL, it is the compiler's responsibility to generate whatever bit pattern the machine uses for that null pointer.