Re: [boost] [Fit] formal review ends 20th March.

Subject: Re: [boost] [Fit] formal review ends 20th March.
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-03-22 12:41:22

On Tuesday, March 22, 2016 at 3:52:23 AM UTC-5, Bjorn Reese wrote:
>
> On 03/21/2016 07:04 PM, Paul Fultz II wrote: 
>
> > However, with lambdas it is the opposite. Lambdas default to const and 
> then 
> > an 
> > explicit mutable keyword is needed for a mutable function object. 
>
> The const-by-default lambdas are the black swans of C++. 
>

Other than working around bugs in MSVC, I have never used a mutable lambda 
in
any of the codebases I have worked on. I find it quite surprising that const
lambdas are considered a "black swan".
 

>
> The fit documentation should at least provide a design rationale for 
> its const-by-default decision which is addressed at those who are 
> unfamiliar with the virtues of immutability. 
>
> Another concern with fit::mutable_ is its name. As you have already gone 
> down the type-traits naming path with fit::decay, it may be an idea to 
> rename fit::mutable_ to fit::remove_const. 
>

No, fit::decay works something like this(ignoring unwrapping references):

template<class T>
std::decay_t<T> decay(T&& x)
{
    return std::forward<T>(x);
}

It simply transforms the type according to the rules of the type trait
std::decay. So, fit::remove_const would imply the same:

template<class T>
remove_const_t<T> decay(T&& x)
{
    return const_cast<remove_const_t<T>>(x);
}

However, fit::mutable_ does not work that way at all. It is an adaptor, so 
it
only accepts a function object, and returns a function object. Even more 
so,it
does not remove const at all, not even from the function object call 
operator.
Rather, fit::mutable_ creates a new function object with const added to the
call operator. However, calling it fit::add_const would imply that it would
prevent mutablilty which is not the case.

So I think fit::mutable_ is a much better name, as it actually describes 
what
is happening to the object using the same terminology that is used for
lambdas. Plus, it the describes the C++ mechanism that is used to implement
fit::mutable_ as well(that is the adaptor uses the mutable keyword).

 

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