Subject: Re: [boost] [mpl] multiset
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2015-02-26 10:28:24
AMDG
On 02/26/2015 05:25 AM, Ganesh Prasad wrote:
> <snip>
> After all TMP is turing complete, hence,
> random access containers should not be non-existent.
>
Turing completeness does not guarantee the
existence of random access. Complexity
classes (i.e. L, P, NP) are invariant
across computation models, but random access
is not a distinct complexity class.
With that being said, mpl emulates random
access like this:
template<class Base, int N, class T>
struct v_item;
template<class Base, class T>
struct v_item<Vector, 0, T> : Base { typedef T item0; };
template<class Base, class T>
struct v_item<Vector, 1, T> : Base { typedef T item1; };
...
This can also be acheived using overload resolution
and decltype:
template<class Base, int N, class T>
struct v_item : Base {
using Base::item;
wrap<T> item(int_<N>);
};
This isn't actually constant time, however, because
of the way most compilers implement overload resolution.
In Christ,
Steven Watanabe