Re: [boost] type<>

From: Samuel Krempp (krempp_at_[hidden])
Date: 2002-06-25 06:16:21

On Thu, 2002-06-20 at 21:14, David Abrahams wrote:

> suggested identity<> before; nobody seemed to get the whole metafunction
> idea, so for the record, I don't care all that much what we call it.

If I think of a "identity" metafunction, I would expect it to follow the
properties of a regular identity :
"x" is perfectly equivalent to  "identity(x)"

But here  "T" is equivalent to "identity<T>::type", 
not just "identity<T>".

For me, it's not identity.. it's a bijective metafunction all-right, but
not identity.
I thought injective metafunctions were usually called "wrappers" -and in
fact those wrappers often are as much bijective as this identity<T>
template. The difference is just that this one is intended to serve as 
as an identity metafunction. (since it's the closest we can get to such
a thing. there is no "let f(T) = T" possible here..)

So, personnally, I'd rather choose for a weaker, but correct, name based
on 'wrapper', 
than a more precise, but slightly incorrect name based on 'identity'..

ah, I thought of a clear manifestation of the difference between a true
identity and this metafunction.
Mathematically, identity composed with itself  is still identity.
So I'd expect 
identity< identity<T> >  to be the exact same thing as
identity<T> 
And, unless you add a specialisation, it's not the case.

Still, the 
T operator()(T x) const { return x; }
really is an identity. 
So if it's included in the struct, it could be named identity<T>, due to
this function (rather than to the struct viewed as metafunction, which
is *not* an identity).

-- 
Samuel