Re: Need to work around g++ 'NULL' warning

"Alf P. Steinbach" <>
Fri, 25 Jan 2008 13:44:18 +0100
* Neil:

  I've constructed a custom smart pointer, and for various reasons I'm
in the situation where I'd like to make the raw-pointer ctor for it

  However, this then disallows assignment from NULL (and implicit
creation, e.g., for 'return NULL' returning a smart pointer). I've
gone out of my way to try to make this smart pointer as drop-in-
replacement friendly as possible, so losing the ability to easily
assign NULL is a bit of a blow.

  I *thought* I had a nice work-around by defining an implicit (int)
ctor, and while VS accepts this, g++ (-Wall -pedantic; probably quite
rightly for most cases) reports a warning:

warning: passing NULL to non-pointer argument ...

  Can anyone think of a clever way of working around this (even if
it's g++-specific)?

  A point of interest may be that '#pragma GCC system_header' can
disable that warning, but only if the actual conversion from NULL to
smart-ptr is in a header with the pragma, and I couldn't figure out
how to wangle that.

  Here's a really basic example:

#include <iostream>

template <typename T>
struct Ptr
    Ptr( const Ptr & ) {} // copy ctor
    explicit Ptr( T * ) {} // only explicit from c-ptr
    Ptr( int ) {} // to catch NULL (don't care about non 0)

int main()
    float f1 = 0.0;

    Ptr<float> p1(&f1); // OK
    Ptr<float> p2(NULL); // warning :-(

    return 0;

struct NullPtr
     template< typename T >
     operator T* () const { return 0; }

     // More stuff if you want it.

NullPtr const nullPtr = NullPtr();

template< typename T >
struct Ptr
     Ptr( Ptr const& ) {}
     explicit Ptr( T* ) {}

int main()
     float f1 = 0;
     Ptr<float> p1( &f1 );
     Ptr<float> p2( nullPtr );
     // Ignore g++ warning of non-usage, or use.

Note: C++0x will introduce some support for typed nullpointers, and it
might be an idea to try to be forward-compatible. If possible.
However, I don't have time to look up the details.

Cheers, & hth.,

- Alf

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