Re: [boost] [type_traits][function_types] Discard param const qualification, bug or feature?

Subject: Re: [boost] [type_traits][function_types] Discard param const qualification, bug or feature?
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-10-05 06:28:26

On Oct 4, 2013, at 5:43 PM, Nevin Liber <nevin_at_[hidden]> wrote:

> On 1 October 2013 05:22, Rob Stewart <robertstewart_at_[hidden]> wrote:
> 
>> No, that should be int x. The reference is a pessimization for int. For larger types, const & is appropriate, though it will apply regardless of the top-level constness of the parameter. IOW, if the type is cheap to
>> copy, copy it, regardless of top-level constness. If it isn't, pass it by const &. That's what call_traits<T>::param_type does for you.
> 
> It's more complicated than that.  When you pass by value the compiler is allowed to assume there are no aliases to your object. Put another way,
> passing by reference can still be a pessimization for types larger than an int (and that is even before copy elision).

Granted, but I know of no way for a library to determine when pass-by-value would be better because of those optimizations.

I suppose you could offer a wrapper type that indicates the desire for pass-by-value, even when call_traits would pass by reference. The developer could try that to see whether profiling shows improvement.

___
Rob

(Sent from my portable computation engine)