Boost.Hana  1.7.1
Your standard library for metaprogramming
boost::mpl::list< T > Struct Template Reference

Description

template<typename ... T>
struct boost::mpl::list< T >

Adapter for Boost.MPL lists.

Modeled concepts

It is possible for MPL lists to model a couple of concepts. However, because they are only able to hold types, they lack the generality required to model concepts like Functor, Sequence and other related concepts.

  1. Comparable
    Two MPL lists are equal if and only if they contain the same number of types, and if all those types are equal.
    // Copyright Louis Dionne 2013-2017
    // Distributed under the Boost Software License, Version 1.0.
    // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
    namespace hana = boost::hana;
    namespace mpl = boost::mpl;
    hana::equal(mpl::list2<int, char>{}, mpl::list<int, char>{})
    );
    hana::not_equal(mpl::list2<int, char>{}, mpl::list<int, char, float>{})
    );
    int main() { }
    Defines macros to perform different kinds of assertions.
    Defines boost::hana::equal.
    constexpr auto equal
    Returns a Logical representing whether x is equal to y.
    Definition: equal.hpp:64
    constexpr auto not_equal
    Returns a Logical representing whether x is not equal to y.
    Definition: not_equal.hpp:54
    #define BOOST_HANA_CONSTANT_CHECK(...)
    Equivalent to BOOST_HANA_CONSTANT_ASSERT, but not influenced by the BOOST_HANA_CONFIG_DISABLE_ASSERTI...
    Definition: assert.hpp:239
    Adapts boost::mpl::list for use with Hana.
    Namespace containing everything in the library.
    Definition: accessors.hpp:20
    Defines boost::hana::not_equal.
  2. Foldable
    Folding a MPL list is equivalent to folding it as a Sequence.
    // Copyright Louis Dionne 2013-2017
    // Distributed under the Boost Software License, Version 1.0.
    // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
    #include <type_traits>
    namespace hana = boost::hana;
    namespace mpl = boost::mpl;
    auto types = mpl::list<long, float, short, float, long, long double>{};
    auto number_of_floats = hana::fold_left(types, hana::int_c<0>, [](auto count, auto t) {
    return hana::if_(hana::trait<std::is_floating_point>(t),
    count + hana::int_c<1>,
    );
    });
    BOOST_HANA_CONSTANT_CHECK(number_of_floats == hana::int_c<3>);
    int main() { }
    Adapts std::integral_constant for use with Hana.
    Defines boost::hana::fold_left.
    constexpr auto count
    Return the number of elements in the structure that compare equal to a given value.
    Definition: count.hpp:41
    constexpr auto if_
    Conditionally return one of two values based on a condition.
    Definition: if.hpp:41
    Defines boost::hana::if_.
    Defines boost::hana::integral_constant.
    Defines boost::hana::plus.
    Defines boost::hana::type and related utilities.
  3. Iterable
    Iterating over a MPL list is just iterating over each of the types it contains, as if it were a Sequence.
    // Copyright Louis Dionne 2013-2017
    // Distributed under the Boost Software License, Version 1.0.
    // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
    #include <type_traits>
    namespace hana = boost::hana;
    namespace mpl = boost::mpl;
    BOOST_HANA_CONSTANT_CHECK(hana::front(mpl::list<int, char, void>{}) == hana::type_c<int>);
    hana::drop_front(mpl::list<int, char, void>{}),
    mpl::list<char, void>{}
    ));
    hana::drop_while(mpl::list<float, double const, int, float&>{},
    hana::trait<std::is_floating_point>),
    mpl::list<int, float&>{}
    ));
    int main() { }
    Defines boost::hana::drop_front.
    Defines boost::hana::drop_while.
    Defines boost::hana::front.
    constexpr auto front
    Returns the first element of a non-empty iterable.
    Definition: front.hpp:32
    constexpr auto drop_while
    Drop elements from an iterable up to, but excluding, the first element for which the predicate is not...
    Definition: drop_while.hpp:44
    constexpr auto drop_front
    Drop the first n elements of an iterable, and return the rest.
    Definition: drop_front.hpp:47
  4. Searchable
    A MPL list can be searched as if it were a tuple containing hana::types.
    // Copyright Louis Dionne 2013-2017
    // Distributed under the Boost Software License, Version 1.0.
    // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
    namespace hana = boost::hana;
    namespace mpl = boost::mpl;
    hana::find_if(mpl::list<int, float, char const*>{}, hana::equal.to(hana::type_c<float>))
    ==
    hana::just(hana::type_c<float>)
    );
    hana::find(mpl::list<int, float, char const*>{}, hana::type_c<void>)
    ==
    hana::nothing
    );
    int main() { }
    Defines boost::hana::find.
    Defines boost::hana::find_if.
    constexpr auto find
    Finds the value associated to the given key in a structure.
    Definition: find.hpp:44
    constexpr auto find_if
    Finds the value associated to the first key satisfying a predicate.
    Definition: find_if.hpp:41
    Defines boost::hana::optional.

Conversion from any <tt>Foldable</tt>

A MPL list can be created from any Foldable. More precisely, for a Foldable xs whose linearization is [x1, ..., xn],

to<ext::boost::mpl::list_tag>(xs) == mpl::list<t1, ..., tn>{}

where tk is the type of xk, or the type contained in xk if xk is a hana::type.

Warning
The limitations on the size of mpl::lists are inherited by this conversion utility, and hence trying to convert a Foldable containing more than BOOST_MPL_LIMIT_LIST_SIZE elements is an error.
// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <type_traits>
namespace hana = boost::hana;
namespace mpl = boost::mpl;
auto xs = hana::make_tuple(hana::type_c<int>, hana::type_c<char>, hana::type_c<double>);
static_assert(std::is_same<
decltype(hana::to<hana::ext::boost::mpl::list_tag>(xs)),
mpl::list<int, char, double>
>{}, "");
int main() { }
Defines boost::hana::to and related utilities.
Defines boost::hana::tuple.