is NULL/false in C and C++ anything more than 0x0

问题:

This question already has an answer here:

• What is the difference between NULL, '\0' and 0 11 answers

As mentioned in the question, I have been using NULL and false(in C++) interchangeably with 0 or 0x0 and so on. I was curious to know if they held any special meaning other than being synonyms of 0.

回答1:

for some platforms NULL is not 0x0

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From Is NULL in C required/defined to be zero? :

NULL is guaranteed to be zero, perhaps casted to (void *)¹.

C99, §6.3.2.3, ¶3

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.(55) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

And note 55 says:

55) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant.

Notice that, because of how the rules for null pointers are formulated, the value you use to assign/compare null pointers is guaranteed to be zero, but the bit pattern actually stored inside the pointer can be any other thing (but AFAIK only few very esoteric platforms exploited this fact, and this should not be a problem anyway since to "see" the underlying bit pattern you should go into UB-land anyway).

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So, as far as the standard is concerned, the two forms are equivalent (!ptr is equivalent to ptr==0 due to §6.5.3.3 ¶5, and ptr==0 is equivalent to ptr==NULL); if(!ptr) is also quite idiomatic.

That being said, I usually write explicitly if(ptr==NULL) instead of if(!ptr) to make it extra clear that I'm checking a pointer for nullity instead of some boolean value.

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1. Notice that in C++ the void * cast cannot be present due to the stricter implicit casting rules that would make the usage of such NULL cumbersome (you would have to explicitly convert it to the compared pointer's type every time).

回答2:

Well, NULL may not be zero, it just usually is. It's dependant on your platform - here are some nasty examples of non-zero NULL machines.

回答3:

NULL has got wide meaning in pointers. Most probably a pointer declared as NULL is something that cannot be referenced but just can be assigned something. whereas 0 or 0x0 can be the value of the pointer which can be both referenced and assigned.

回答4:

Generally in C++ you shouldn't use NULL. In C, NULL is a macro for (void*)0, whereas in C++ you should use 0 instead of NULL. But you can't use false instead of 0 for pointers! Apart from that, they are actually the same, sometimes causing confusion.
This is why in C++11 a nullptr was defined as use for pointers.

回答5:

In C and historic C++, NULL has to be a zero-valued integer constant, typically 0. I think that C can include an explicit cast to void*, but C++ can't since that language doesn't allow implicit conversion from void* to other pointer types.

In modern C++, it could be either nullptr instead.

In either case, any zero-valued integer constant (including oddities such as '\0' and false) is convertible to a null pointer value.

回答6:

from linux kernel stddef.h

#define NULL ((void *)0)

enum {
        false   = 0,
        true    = 1
};

So it is not exactly false

even though this post claims it is NULL , 0x0 and false are almost the same.