CODEWITHSUNDEEP
c++cpointersnull
Howard Wang
Howard Wang

Reputation: 472

Why can pointers be NULL

In C or C++, isn't NULL just the constant integer 0? Why are we allowed to assign some pointer to the value NULL if it a constant integer? Wouldn't a compiler say that the two types don't match?

Upvotes: 2

Views: 585

Answers (3)

John Bode
John Bode

Reputation: 123598

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.66) 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.
66) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19.

C 2011 online draft

A literal 0 in a pointer context is treated specially by the compiler, and is understood to be the null pointer constant. In the context of your source code, it behaves like any other zero-valued expression.

However, once your code is translated to machine code, all occurrences of the null pointer constant will be replaced with whatever value the underlying platform uses to represent a well-defined invalid pointer value, whether that’s 0 or 0xFFFFFFFF or 0xDEADBEEF.

Upvotes: 1

Barmar
Barmar

Reputation: 782683

The interpretation of the constant 0 depends on the context. If it's being used in a context where a number is required, it's the number zero. If it's being used in a context where a pointer is required, it's treated as a null pointer. So if you have the code:

int n = 0; // assigns zero
int *p = 0; // assigns null pointer
somefunc((char *)0); // passes null pointer to the function

In C, the macro NULL is permitted to expand into either an integer constant 0 or such a constant cast to void *. In C++ pre-11, it can expand into either an integer constant that evaluates to 0. In C++-11 it can expand into an integer literal with value 0 or a prvalue of type std::nullptr_t (e.g. nullptr).

See http://en.cppreference.com/w/cpp/types/NULL for the C++ description, http://en.cppreference.com/w/c/types/NULL for the C description.

Upvotes: 4

Steve Summit
Steve Summit

Reputation: 48083

There are two issues:

  1. A type can have a "special", out-of-band, exceptional value. For example, floating-point numbers have the value NaN. And pointers have a special, not-a-valid-pointer value, the null pointer.

  2. What's the name of the null pointer? In C, for better or worse, its name is 0. (It would avoid all kinds of confusion if its name were a separate keyword like null or nil, but that's not the way the language definition came out.)

But yes, you're right, the ability to assign the constant 0 to a pointer variable is a special case, enshrined in the language definition.

Upvotes: 2

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