Re: [boost] [type_traits] Rewrite and dependency free version

Subject: Re: [boost] [type_traits] Rewrite and dependency free version
From: Peter Dimov (lists_at_[hidden])
Date: 2015-01-14 19:51:44

John Maddock wrote:
> I'm going to throw this out there to see what folks think:

They very much approve of, and thank you for, your work. :-) This is so much 
better. :-)

> In a branch called "Version2", see 
> https://github.com/boostorg/type_traits/tree/Version2 is a partially 
> rewritten type_traits lib: it doesn't yet contain all the traits of the 
> old one, but there's a pretty good selection to test.

I wonder why you have decided to return a reference to mpl::bool_, instead 
of just ("stupidly") returning by value. Compilers like "stupid" code, they 
optimize it better. Typically, the use case is:

void f( mpl::true_ );
void f( mpl::false_ );

f( is_pointer<X>() );

and if you return by value, it can construct directly into the argument, 
whereas by reference it needs to use the copy constructor.

That said, I still think that it may be better to add the conversion on the 
MPL side; this will obviate the need for the fragile forward declarations 
(and it will support any integral constants, not just this one).

We can, of course, in addition add a converting constructor to 
integral_constant as well, so that the "officially blessed" way to dispatch 
would be on true_type and false_type. This dispatch will work on std:: 
traits as well.