[note: forwarded to C++ committee -sdc ]

In the following code, I expect both "null" and "FALSE" to be null pointer
constants -- and that the code should compile and output the string "int*"
twice to cout:

#include <iostream>

using namespace std;

void foo(int* p)
{
    cout << "int*" << endl;
}

int main(void)
{
    const int null = 0;
    foo(null);
    const bool FALSE = false;
    foo(FALSE);
}

ISO/IEC 14882-1998 4.10 states:

"An  integral constant expression rvalue of integer type that evaluates to
zero (called a null pointer constant)  can  be  converted  to  a  pointer
type."

Stroustrup appears to agree with me -- he states (3rd edition page 88):

"In C, it has been popular to define a macro NULL to represent the zero
pointer. Because of C++`s tighter type checking, the use of plain 0, rather
than any suggested NULL macro, leads to fewer problems. If you feel you must
define NULL, use:
  const int NULL = 0;"

However gcc 3.3.1 rejects this code with the errors:

    bug.cc:17: error: invalid conversion from `int' to `int*'
    bug.cc:19: error: cannot convert `const bool' to `int*' for argument `1'
to `
        void foo(int*)'

I have reported this as a bug
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13867), but the gcc team states
that 4.10 requires that a null pointer constant must be an rvalue -- and no
implicit conversion from an lvalue to an rvalue is required
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=396):

"a null pointer constant is an integral constant expression rvalue that
evaluates to zero [4.10/1] in this case `null' is an lvalue. The standard
does not specify that lvalue->rvalue decay happens here, so `null' is not a
null pointer constant."

I disagree with the gcc teams interpretation -- I don't see why 3.10 doesn't
apply:

"Whenever  an  lvalue appears in a context where an rvalue is expected,  the
lvalue is converted to an rvalue;"

The insertion of the word rvalue appears to have occurred during
standardization -- it is not present in either Stroustrup 2nd edition or the
3rd edition. Does the committee deliberately intend to exclude an lvalue as
a null pointer constant by adding the word rvalue? If so, it leads to the
rather bizarre fact that "null" is not a null pointer constant, but "null +
0" is!

Lloyd Lewins



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