[C++-sig] Re: return_self_policy

[David Abrahams]

Nikolay Mladenov <nickm at sitius.com> writes:

> David Abrahams wrote:
>> 
>> Nikolay Mladenov ?nickm at sitius.com? writes:
>> 
>> ? Posting return_self policy implementation
>> ?
>> ? Nikolay'''
>> 
>> Nikolay,
>> 
>> This is wonderful!  Now, I hate to do this, but I just realized that
>> this should really be generalized to something which takes an
>> argument number as its parameter and returns that argument:
>
> I have already thought about it (I expected it from you ;-) )
> and it is already there in some form:
> the definition of return_self_policy is
>
> 	template<class Base>
> 		struct return_self_policy : 
> 			detail::return_arg<0, Base> {}
>

Oh, wonderful! Very shrewd of you to anticipate me.

>> 
>>  return_identity?0?                     // error
>>  return_identity??, return_identity?1?  // same as return_self_policy
>>  return_identity?2?                     // return the 2nd argument
>>  return_identity?3?                     // return the 3rd argument
>>  ...
>> 
>> etc.
>
> So return_arg is as your return_identity, although return_arg<0> is not
> an error but return_self.

Well, y'see, by convention in call policies, zero is used to refer to
the return value and 1 refers to the first argument.  See
with_custodian_and_ward.

>> Don't you think that makes more sense?  Would you mind making this
>> modification?
>
> I agree that it makes more sense, but I am not sure how much the
> "more" is.

:-)

> Generally I don't mind.
>
>> 
>> Thoughts, objections, screaming...?
>
> My question is: why start counting from 1? This will make the code more
> complicated and difficult to read.

Consistency.  It's a convention in CallPolicies.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com