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Comment by edoo | Hacker News Reader

Nulls in strongly typed languages can get rather weird but from a C/C++ perspective it is the same as 0. nullptr is just a correctly casted 0.

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That's not correct.

In C, the literal "0" is a null pointer constant handled at the compilation stage, but casting a runtime zero is not specified to yield a null pointer (and the address zero can be perfectly valid and usable), nor are null pointers specified to be zero-valued (quite the opposite).

I disagree. nullptr is directly convertable to a boolean 0:

#include <stdio.h>

int main(int argc, char *argv[]) {

  printf("%d\n", nullptr == 0);

  return 0;
}

1. You're comparing a null pointer literal with a null pointer constant. You're literally comparing two things guaranteed to be equal. How, exactly, is it surprising that they do? Did you even attempt to understand what I wrote?

2. The specific implementation you have on hand could use zero-valued null pointers, I'm telling you what the standard doesn't say, confirmed by the C FAQ:

http://c-faq.com/null/varieties.html

http://c-faq.com/null/confusion4.html

http://c-faq.com/null/machexamp.html

3. The null pointer is not "directly convertible to a boolean 0", it's the literal 0 which expresses a null pointer: http://c-faq.com/null/ptrtest.html

I see what you are saying. I mean zero as null constant. I've never even seen anyone try to assign a null value with some integer zero at some memory address. Using the NULL constant in C/C++ is near effortless and 0 just works no matter what the underlying implementation is doing. So for all intents and purposes nullptr is just a casted NULL (0) constant.

This is only true in C, which is part of why C is less insane than C++.

The two languages should not be conflated anymore.

This is not even true in C. 0 is a "null pointer literal" when used in pointer context, this does not imply that the actual null pointer has a value of zero.

Ah yes you are right, an interesting point however,

    6.3.2.3 Pointers
    
       3. An integer constant expression with the value 0, or such an expression cast to type void *,
          is called a null pointer constant. 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.
    
       4. Conversion of a null pointer to another pointer type yields a null pointer of that type.
          Any two null pointers shall compare equal.

Looking at the definition of NULL:

    7.17 Common definitions <stddef.h>
    
        3 The macros are
        
                NULL
       
          which expands to an implementation-defined null pointer constant;

Meaning that NULL is implementation defined, but is assured by the standard to compare equal to any 0-valued pointer of any type, even if it is itself another special value.

Saying “equal to any 0-valued pointer” there can be misleading. It is true of a pointer assigned from a (foo*)0 constant, but not true of a pointer to a hardware address 0 or a pointer with bits all 0 (assuming they exist).

Actually nullptr is a struct.

reminds that https://en.wikipedia.org/wiki/Blind_men_and_an_elephant :)

nullptr is actually "a prvalue of type std::nullptr_t" or something like that. Since C++11 NULL is the same as nullptr.

And it is directly convertible to a bool. The primary effectiveness seems to be with operator overloading where a NULL could trigger an integer method instead of a pointer method.

Yes that's the example given here. https://en.cppreference.com/w/cpp/language/nullptr So its an improvement but not exactly a game changer.

To the small extent to which "since C++11" is even relevant to "from a C/C++ perspective", even in C++11 NULL is still allowed to simply be defined as 0.

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