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Carsten Fuchs | 5 Aug 2012 10:40
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[intrusive_ptr] Why are comparison operators not friends?

Hi all,

I was just (re-)reading item 46, "Define non-member functions inside templates when type 
conversions are desired", in Scott Meyers "Effective C++, 3rd edition", and was 
wondering how and why the comparison operators for intrusive_ptr work when used with 
NULL, even though they're not friend functions as described in Scott Meyer's item 46.

That is, the example program below compiles and links fine both with GCC 4.6 as well as 
with Visual C++ 2010 -- but how does do the compilers match the == operator, whose right 
operand is NULL, an int value?

If I try to use the exact same templated binary operators like this in my own classes, 
and want NULL matched to one of the T* or U* operands, then I have to do it with friend 
functions or else the compilers don't find a match.

Why does it work here?

Best regards,
Carsten

#include <boost/intrusive_ptr.hpp>
#include <iostream>

class CRefCounted
{
     public:

     CRefCounted() : RefCount(0) {}

     long RefCount;
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Hite, Christopher | 9 Aug 2012 12:47

Re: [intrusive_ptr] Why are comparison operators not friends?

> I was just (re-)reading item 46, "Define non-member functions inside templates when type conversions
are desired", in Scott Meyers "Effective C++, 3rd edition", and was wondering how and why the comparison
operators for intrusive_ptr work when used with NULL, even though they're > not friend functions as
described in Scott Meyer's item 46.
I believe you're talking about the ancient technique of friend injection used here for example:
http://en.wikipedia.org/wiki/Barton%E2%80%93Nackman_trick

The modern good way of doing this Koenig lookup which I think deprecates that trick.
http://en.wikipedia.org/wiki/Argument-dependent_name_lookup

Just define the operator== in the namespace of the type it works on.  It'll find that name in the first lookup
stage and then look for a fitting overload.

In your code the first one is taken.  Though I'm not sure what U is; I'm guessing void.

template<class T, class U> inline bool operator==(intrusive_ptr<T> const & a, U * b);
template<class T, class U> inline bool operator==(T * a, intrusive_ptr<U> const & b);

Chris

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Carsten Fuchs | 10 Aug 2012 10:25
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Re: [intrusive_ptr] Why are comparison operators not friends?

Hi Christopher,

Am 2012-08-09 12:47, schrieb Hite, Christopher:
>> I was just (re-)reading item 46, "Define non-member functions inside templates when type conversions
are desired", in Scott Meyers "Effective C++, 3rd edition", and was wondering how and why the comparison
operators for intrusive_ptr work when used with NULL, even though they're > not friend functions as
described in Scott Meyer's item 46.
> I believe you're talking about the ancient technique of friend injection used here for example:
> http://en.wikipedia.org/wiki/Barton%E2%80%93Nackman_trick
>
> The modern good way of doing this Koenig lookup which I think deprecates that trick.
> http://en.wikipedia.org/wiki/Argument-dependent_name_lookup

Thank you very much for your reply!

I still have to experiment and compare and check my code, but the linked articles and 
their references help a lot!
Thanks!

Best regards,
Carsten

--

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    Cafu - the open-source Game and Graphics Engine
for multiplayer, cross-platform, real-time 3D Action
           Learn more at http://www.cafu.de

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