Re: [MPL] Comments for a newbie

"cj2048" writes:

Hello everyone,

I've only recently taken my first small steps into the world of metaprogramming, and now I've hit a snag. I'm trying to create a metafunction that returns the smallest unsigned integral type with at least the specified number of bits and, if no such type exists, reports an error.

For example, ceil_uint<24> (named so for lack of a better name) returns the first member of the list consisting of

unsigned char short unsigned int unsigned int long unsigned int

whose number of bits is greater than or equal to 24.

The easy answer is http://www.boost.org/libs/integer/integer.htm But assuming you want to do this yourself...

Now, ceil_uint<N> seems to work fine for 0 <= N <= 32, but with N outside this range, g++ (3.2) gives me a compilation error:

[cej@kirk aux]$ g++ -I/usr/include/boost test.cpp -o test test.cpp: In function `int main()': test.cpp:48: conversion from `int' to non-scalar type `boost::mpl::void_' requested test.cpp:49: no match for `-- boost::mpl::void_&' operator test.cpp:50: no match for `boost::mpl::void_& == unsigned char' operator test.cpp:52: no match for `boost::mpl::void_& == short unsigned int' operator test.cpp:54: no match for `boost::mpl::void_& == unsigned int' operator

I guess that a compilation error is desirable in these cases, but is there some MPL-blessed way to make the error message(s) more informative (i.e., tell the user that N is outside the valid range)?

Not yet. BOOST_STATIC_ASSERT() is a pretty good solution, but it doesn't put any nice text like "out of range" in your error message.

I'd greatly appreciate any and all suggestions for improving the metafunction. I'd also like to know the generally recommended way to separate the "invocation" of the metafunction from its definition.

Use separate headers? Sounds too obvious; you must be after something else...

I intend to invoke the metafunction in a header file like this:

namespace X { /* a definition of ceil_uint here would clutter the interface of class Y */ template<int N> class Y { /* ... */ typename ceil_uint<N>::type data; // invocation }; }

Having the definition and the invocation in the same header file seems to clutter the interface of the class.

/* test program */

#include "boost/mpl/vector.hpp" #include "boost/mpl/find_if.hpp" #include "boost/mpl/transform.hpp" #include "boost/mpl/multiplies.hpp" #include "boost/mpl/sizeof.hpp" #include "boost/mpl/greater_equal.hpp" #include "boost/mpl/int.hpp" #include "boost/mpl/at.hpp" #include "boost/mpl/distance.hpp"

#include <limits> #include <iostream>

namespace mpl = boost::mpl; using namespace mpl::placeholders;

First point: MPL metafunctions operate on types, not numbers. To stay consistent, pass integral type wrappers like int_<N>.

template<int N> struct ceil_uint { typedef mpl::vector< unsigned char, short unsigned int, unsigned int, long unsigned int

...

types;

typedef typename mpl::transform< types, mpl::multiplies< mpl::sizeof_<_>, mpl::int_ >

...

::type result;

Second point: I don't think this is consistent with the standard. Does the standard guarantee that an unsigned integral type of size M has M*CHAR_BIT usable bits?

typedef typename mpl::find_if< result, mpl::greater_equal< _1, mpl::int_<N> >

...

::type iter;

You could BOOST_STATIC_ASSERT((!is_same::value)) right here.

typedef typename mpl::at< types, typename mpl::distance< typename mpl::begin<result>::type, iter >::type

...

::type type; };

Hmm, well it's a very inefficent way to get where you're going because you have to construct and traverse several type sequences. How about: template <class T> struct digits : mpl::int_ {}; template <class N> struct ceil_uint : typename mpl::find_if< mpl::vector< unsigned char, short unsigned int, unsigned int, long unsigned int > , mpl::greater_equal, N> >::type {}; ?? If you want to check for errors, you can use mpl::aux::has_type to check whether ceil_uint is valid: BOOST_STATIC_ASSERT(mpl::aux::has_type::type::value); and if you want a nice error message, do something like: number_of_bits_out_of_range(mpl::true_); ... enum { whatever = sizeof( number_of_bits_out_of_range(mpl::aux::has_type::type) ) }; HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com