Re: Does the null-pointer-constant occupy storage?

From:

Francis Glassborow <francis.glassborow@btinternet.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 6 Aug 2009 08:17:06 CST

Message-ID:

<8O6dnfl8wZ8llefXnZ2dnUVZ8qCdnZ2d@bt.com>

/dev/null wrote:

Consider the following program.

#include <stdio>

const int NULL = 0; // Recommended NULL definition according to The C+
+ Programming Language by Bjarne Stroustrup.

There is no reason that NULL should take up storage unless you take the
address of it.

// function that changes NULL.
void foo( void )
{
int *p = const_cast<int32 *>(&NULL);

Here you take the address but then try to apply a const_cast to
something that is declared as const. The result of then trying to write
to that storage is undefined behaviour. const_cast does not change
read-only to mutable all it is allowed to do is to remove logical
constness (normally acquired via a reference or pointer.

*p = 42;

Undefined behaviour

}

// Print value of NULL.
void print_null( void )
{
     std::cout << NULL;
}

int main( void )
{
     foo();
     print_null();
}

Is this a well-formed C++ program?
If so, what will it print?

Best regards,
/J

Incidentally NULL is NOT the null pointer constant it is just exactly
what it is declared to be, an immutable int whose value is zero. It is
the context that can make 0 a null pointer constant.

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