Re: [boost] determining if an object is faster to pass by value or const reference

From: John Maddock (john_at_[hidden])
Date: 2008-07-04 04:23:33

Michael Marcin wrote:
>> The documentation makes no mention of allowing extension for user
>> defined types. It in fact has a table which clearly states that
>> param_type is a const reference for all user defined types.
>>
>> In a specific case I was working on a 3d renderer that we used for a
>> mobile project where we used a templated fixed point class. There
>> were
>> some low level functions that worked on a typename Real which used
>> pass
>> by const reference because cheap copy was not a requirement of our
>> Real concept. This project happened to only be using this fixed
>> point type
>> which was a wrapper over int. We branched the code and changed the
>> functions to pass by value. This resulted in a measurable performance
>> increase.
>>
>> I would like to take classes like this and specialized param_type for
>> them but the documentation for this library doesn't seem to allow
>> for that.

There's nothing to stop you from adding a full or partial specialisation of 
call_traits to your header:

struct my_value_type{ ... };

namespace boost{

template<>
struct call_traits<my_value_type>
{
  typedef my_value_type value_type;
  typedef my_value_type& reference;
  typedef my_value_type const& const_reference;
  typedef my_value_type param_type;
};

} // namespace

Then once your functions are call_traits aware, it's then easy to tweek 
behaviour by adding specialisations as required.

Does this help?

John.