Re: [Boost-users] Need boost's help with poor design

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-03-26 14:36:13

AMDG

Robert Dailey wrote:
> The term 'inherit' here does not refer to class inheritance obviously. 
> It's an unusual usage of the term to me. When you do 
> inherit<empty_base, WalkPacket> what exactly is that doing?

template<class T1, class T2>
struct inherit : T1, T2 {};

> Is it joining them in a list container or something? What is the 
> resulting type of mpl::inherit_linearly::type? Is the result of 
> mpl::inherit_linearly::type implicitly constructible from an 
> mpl::transform::type?

The result of the inherit_linearly call inherits using multiple 
inheritence from
all the types in the sequence.

> I'm going to read over the docs a few more times in the meantime to 
> see if I can grasp this concept. Also, I'm not seeing how you're 
> filling in the 2 placeholders in the signal_holder typedef.

mpl::inherit_linearly treats the second argument as a binary lambda 
expression.
It is very similar to the runtime std::accumulate.  Think of inherit<_, 
_> as
the MPL equivalent of a function object that returns inherit<T1, T2> for
arguments T1 and T2.

> Also, mpl::vector has a max size limit I'm sure. Right now we have a 
> very minimal set of packet types, but in the future we have to 
> consider the possibility that 100+ packet types will exist (the final 
> number is ultimately unknown). If this is true, I doubt mpl::vector 
> would be usable anymore since 100 seems too large for it. This makes 
> me feel like I'm going to have to resort to a completely template-less 
> approach. I might have to send a generic Packet object to all 
> subscribers, which has a ::Get() method that performs a 
> static_cast<>() to the concrete type based on an ID.

MPL allows configuration to make vector able to hold larger numbers of 
types.
If you need such a large number of Packets, compilation may be very slow
with metaprogramming.

In Christ,
Steven Watanabe