This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See for the official list.


456. Is initialized const int or const bool variable a null pointer constant?

Section: 7.3.12  [conv.ptr]     Status: NAD     Submitter: Lloyd Lewins     Date: 31 Jan 2004

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;
     const bool FALSE = false;

ISO/IEC 14882-1998 7.3.12 [conv.ptr] 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: error: invalid conversion from `int' to `int*' error: cannot convert `const bool' to `int*' for argument `1' to `
         void foo(int*)'

I have reported this as a bug (, 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 (

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 7.2.1 [basic.lval] 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!

Notes from the March 2004 meeting:

We think this is just a bug in gcc. The const variable does get converted to an rvalue in this context. This case is not really any different than cases like

  const int null = 0;
  int i = null;
  const int i = 1;
  int a[i];
(which are accepted by gcc). No one would argue that the second lines of those examples are invalid because the variables are lvalues, and yet the conversions to rvalue happen implicitly for the same reason cited above -- the contexts require an rvalue.