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Subject: Re: [boost] Need for BOOST_PP_VARIADIC_POP_FRONT
From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2011-11-14 14:53:47

On Mon, 14 Nov 2011 10:57:27 +0000, Gennadiy Rozental wrote:

> As for scaling I am not sure what you mean. Would you care to show an
> example?

Here's the list that I gave Eric when he asked me previously in regards to
variadic templates, but really these are true for all forms of variadics.

1) If forces a particular argument order. It either tends to break
symmetries or tends to require argument orders to change on many things.
I.e. not only is the argument order forced for the variadic interface, it
tends to be viral and therefore a maintenance disaster.

2) It does not play well with overloading (via number of arguments with
macros, specialization of templates, or function overloading).

3) It frequently destroys the ability to use default arguments.

4) It does not work when multiple data structures need to be passed.

5) It removes the ability to use the single variadic data for other stuff.

6) Non-delimited data structures such as this cannot nest.

7) In the particular context of macros, it cannot contain elements that
contain "open" commas such as class template instantiations with multiple
arguments. Currently, sequences in the pp-lib cannot do this either, but
that is the result of their current definition. Non-unary sequences (such
as (a, b)(c, d)(e, f)) can be specified and truly variadic sequences (such
as (a)(b, c)(d, e, f)) can also be specified (the difference relates to
the way that the "elements" of the sequence need to be passed on to other

The best ways that variadics can be used is for initialization of some
kind or for "functions" that have no depth (i.e. do not rely on other
variadic public or private interfaces--e.g. things like min and max).
Variadics can also be used effectively as a mechanism to pass "opaque"
argument sets through other algorithms or through other functions (e.g.
perfect forwarding). Of particular note in this latter case is that the
actual (ultimate) target of those arguments is not variadic which implies
that from a user standpoint, it outer interface isn't truly variadic only
certain numbers and types of arguments will usually work. The effect is
simply an implementation mechanism to forward to an overload set without
replicating the overload set. This has slightly less context in C++11 due
to lambdas' ability to easily encode such information directly.

Paul Mensonides

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