这些空指针，还是地址为0的指针？

Question

If I write

int zero = 0;

void *p1 = (void *)0;
void *p2 = (void *)(int)0;
void *p3 = (void *)(0 /*no-op, but does it affect the next zero?*/, 0);

void *p4 = (void *)zero;    // For reference, this is a pointer to address zero
void *p5 = 0;               // For reference, this is a null pointer
void *p6 = NULL;            // For reference, this is a null pointer
void *p7 = nullptr;         // For reference, this is a null pointer (C++11)

static const int static_zero_1 = 0;       // Is this a literal zero when used?
static const int static_zero_2 = 1 - 1;   // No "literals 0" per se... is it?
void *p8 = (void *)static_zero_1;   // I have seen weird substitution rules...
void *p9 = (void *)static_zero_2;   // do they apply for NULL too?

which of p1 , p2 , and p3 ( edit: I added p8 and p9 ) would be null pointers (ie == NULL , may or may not be address zero), and which of them would be pointers with the address zero (may or may not be == NULL )?

If the answer is different in C and C++, what is it in each of them?

Answer 1

And to make Andy's answer complete with C:

From the C99 Standard :

6.3.2.3 Pointers

1 A pointer to void may be converted to or from a pointer to any incomplete or object type. A pointer to any incomplete or object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

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.

So any integer constant expression which evaluates to 0 is a null pointer constant and can be converted to a NULL pointer. Effectively in your example, all pointers through except p4 , p8 and p9 are null pointers. p4 , p8 and p9 need not be null pointers since their initialization is not a constant expression because it contains variables (even if const qualified).

Here's another answer about NULL in C++ , for the record.

Answer 2

which of p1 , p2 , and p3 would be null pointers?

In C++11, all of them. Per paragraph 4.10/1 of the C++11 Standard:

A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t . [...]

Therefore, according to the terminology of the Standard, everything which is a constant (integral) expression and evaluates to 0 is a null pointer constant (not a null pointer , yet). The only one which is not a constant expression that evaluates to 0 or a prvalue of type nullptr_t in your example is zero , because it is not a constant expression.

The paragraph continues:

A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type. Such a conversion is called a null pointer conversion . Two null pointer values of the same type shall compare equal.

So in your example all the pointers except p4 are null pointer values and compare equal among themselves.

Answer 3

p1 and p2 are null pointers; p3 is implementation defined, and may be something else. (A comma operator cannot be part of a constant expression. And the mapping of a non-constant integral value 0 to a pointer is implementation defined.) C is identical to C++ here.

p8 and p9 are both null pointers in C++, but not in C.

With regards to your comment on static_zero_2 , there is no requirement in either language that a literal zero be present, anywhere. g++ defines NULL as the compiler built-in __null , for example, and you can use (1 - 1) , or '\\0' , or any other constant expression evaluating to 0.