相比之下，我可以假设 NULL 值为假吗？
Can I assume NULL value in comparison as the false?
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所有 (any_type *):
都是等价的
any_type *val;
if (val) { ... };
和
if (val != NULL) { ... };
是的。你可以这样使用宏NULL。它是一个空指针常量，等于 0。
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.
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.
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所有 (any_type *):
any_type *val;
if (val) { ... };
和
if (val != NULL) { ... };
是的。你可以这样使用宏NULL。它是一个空指针常量，等于 0。
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
expris any expression at all, the compiler essentially acts as if it had been written asif((expr) != 0)Substituting the trivial pointer expression
pforexpr, we haveif(p) is equivalent to if(p != 0)and this is a comparison context, so the compiler can tell that the (implicit)
0is 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.
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
0orNULL, it is the compiler's responsibility to generate whatever bit pattern the machine uses for that null pointer.