On Sun, Nov 06, 2011 at 12:17:01PM -0900, Dave Abrahams wrote:

on Sun Nov 06 2011, Robert Jones wrote:

...

The shared_ptr docs suggest that a smart pointer employing a null deleter can be implemented using a functor. Why prefer a functor to a function?

Two reasons I can think of:

- Function objects can have a templated operator(), so you don't need to write a new one for each T in shared_ptr<T>.

- One less level of function call indirection (very minor win)

Let's not forget a very strong reason - functors can contain arbitrary state, as that provided when binding arguments to a callable with Boost.Bind, or any user-defined functor or stateful lambda. A naked free function pointer should really not appear in a modern C++ interface, it's something you expect to see in a C-style API, with some void* blob of "context" passed in if very lucky. -- Lars Viklund | zao@acc.umu.se

Robert Jones wrote:

Thanks Gents - The phrasing of my question was hopelessly lax: I appreciate in the general case why one might choose to use a functor rather than a function as a callable object, what I didn't grasp (and there may be nothing to grasp), was why this (from the shared_ptr docs)

struct null_deleter { void operator()(void const *) const { } };

Is implemented as a functor rather than a function.

There's nothing to grasp. The only difference is that sizeof( null_deleter ) is 1, whereas sizeof the function would be 4 or 8, but due to alignment, they both will take up the same space in the actual control block, so it doesn't matter one bit. Using a functor here is purely coding style - the size difference, the fact that operator() would be inlined, the ability to template operator() could all have been relevant in another context.

on Mon Nov 07 2011, "Peter Dimov" wrote:

Robert Jones wrote:

...

Thanks Gents - The phrasing of my question was hopelessly lax: I

...

appreciate in the general case why one might choose to use a functor rather than a function as a callable object, what I didn't grasp (and there may be nothing to grasp), was why this (from the shared_ptr docs)

struct null_deleter { void operator()(void const *) const { } };

Is implemented as a functor rather than a function.

There's nothing to grasp. The only difference is that sizeof( null_deleter ) is 1,

...or effectively zero if EBO is leveraged. -- Dave Abrahams BoostPro Computing http://www.boostpro.com