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From: Arkadiy Vertleyb (vertleyb_at_[hidden])
Date: 20040701 22:17:08
"Dan" <dan_at_[hidden]> wrote
> How does this _1 and _2 stuff work anyway?
>From my experience, in order to understand this stuff, you first have to
temporarily forget about _1 and _2 (unless you are already fluent with
Boost.Lambda).
An MPL algorithm (fold, iter_fold, etc.) actually accepts 3 parameters:
1) sequence;
2) initial type (state);
3) metafunction class.
The third parameter defines how to create a new state from the old state and
the current element (or iterator) of the sequence, for your example
something like this:
struct adder
{
template<class State, class Current>
struct apply
{
typedef typename mpl::plus<State, Current>::type type;
};
};
(this may not compile  I am not sure about the order of template
parameters)
The adder is analogous to a functor in STL, and apply is analogous to the
function call operator.
Having defined this "functor", you can get your result something like this:
typedef mpl::fold<MySequence, mpl::int_<0>, adder>::type MyResult;
As far as _1, _2, etc., is concerned  this is MPL Lambda facility, it
allows to define metafunction classes right inplace, in the same way
Boost.Lambda does for runtime functors. I believe (I may be wrong of
course) that MPL Lambda is more complicated than the rest of MPL, but, in
short, _1 stands for the first parameter of apply, and _2  for the second
parameter.
When you write:
mpl::plus<mpl::_2, mpl::_1>
this effectively (by the means of MPL Lambda) defines a metafunction class
("functor") that acts in the same way as the "adder" does.
HTH,
Arkadiy
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