[Boost-users] Re: Exercice 3-7 (C++ Templates Metaprogramming)

Philippe Mori writes:

I do not understand well the semantic of some constructs.

1) For t1,

Namely, typedef mpl::lambda >::type t1;

does the external mpl::lambda has any effect on some use of t1

Yes. 'lambda' metafunction itself doesn't have a special status in, well, lambda expressions, and the above doesn't behave any differently from the following rewrite: template< typename T > struct f : lambda<T> {}; typedef mpl::lambda< f<_1> >::type t1;

or mpl::lambda<_1>::type and also mpl::lambda<_1> are always equivalent?

Since 'lambda' is a regular metafunction, 'lambda<_1>' and 'lambda<_1>::type' are not equivalent. The former is a nullary metafunction, and the latter yields the result of the invocation, which happens to be '_1'.

2) Does the uses of lambda in t8

Namely, typedef mpl::apply<_1,mpl::lambda< mpl::plus<_1,_2> > >::type t8;

has any effect or the result is always the same as with t2.

't2', that is typedef mpl::apply<_1,mpl::plus<_1,_2> >::type t2; is simply identical to 'mpl::plus<_1,_2>' (which you can verify using BOOST_MPL_ASSERT). Likewise, 't8' is identical to 'mpl::lambda< mpl::plus<_1,_2> >'.

3) Does there are other tests that the ones below (i to vii) that would shows some other differences in the semantics of the expression?

Yep. Test cases in "$BOOST_ROOT/libs/mpl/test/apply.cpp" (http://cvs.sourceforge.net/viewcvs.py/boost/boost/libs/mpl/test/apply.cpp?rev=1.5&view=markup) assert many of the expressions appearing in the exercise + a few other corner cases.

4) Same question but for to_be_filled part of my tests. Anything different that the four ones belows (a to d)?

Ignoring the fact that some combinations won't make much sense or won't compile (e.g. 'display_type(mpl::apply::type'), 'to_be_filled' part won't influence general behavior of any of these constructs.

5) In any ot the tests with a to_be_filled part, is it possible to have more than one template argument and that the extra ones are used?

With some of t's, e.g. 't2', you'd _have_ to provide a second argument for the 'apply' expression(s) to compile. In general, both lambda and bind expressions allow for an arbitrary number of extra arguments (<= BOOST_MPL_LIMIT_METAFUNCTION_ARITY, of course) which are discarded (similarly to Boost.Bind).

The thing I have some of the following (what I have tried and what compile depends on the construct - that is t1, t2,...) :

i) display_type(tn);

ii) display_type(tn::type); iii) display_value(tn::type::value);

iv) display_type(mpl::apply::type) v) display_value(mpl::apply::type::value)

vi) display_type(tn::apply::type) vii) display_value(tn::apply::type::value)

where tn is one of the type t1, t2,... t8 and to_be_filled is something similar to one of those :

a) int b) X where X is a metafunction class c) mpl::plus, int_<2> > d) Y where Y is a simple metafunction similar to always_int in section 3.5.4.

I have look at the documentation but it is hard to see the relation between each construct, the possible uses of each ones and their prmary purpose. I would be nice to provide more samples and more equivalences.

Hope the above helps! Sorry for the late reply, -- Aleksey Gurtovoy MetaCommunications Engineering