hana/test/comparable/defaults.cpp

/*
@copyright Louis Dionne 2014
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 <boost/hana/comparable/comparable.hpp>

#include <boost/hana/core/datatype.hpp>
#include <boost/hana/core/instantiates.hpp>

#include <boost/hana/detail/static_assert.hpp>
using namespace boost::hana;


struct integer {
    int value;
    friend constexpr bool operator==(integer a, integer b)
    { return a.value == b.value; }
};

struct integer2 {
    int value;
    struct hana_datatype;
    friend constexpr bool operator==(integer2 a, integer2 b)
    { return a.value == b.value; }
};

using Integer = datatype_t<integer>;
using Integer2 = datatype_t<integer2>;

int main() {
    // An instance is provided for EqualityComparable types whose data type
    // is the same as their C++ type.
    BOOST_HANA_STATIC_ASSERT(equal(integer{0}, integer{0}));
    BOOST_HANA_STATIC_ASSERT(!equal(integer{0}, integer{1}));
    BOOST_HANA_STATIC_ASSERT(instantiates<Comparable, Integer, Integer>);

    // No instance is provided when the data type differs from the C++ type,
    // even if the C++ type is EqualityComparable.
    BOOST_HANA_STATIC_ASSERT(!instantiates<Comparable, Integer2, Integer2>);

    // Two objects of different data types are unequal by default.
    struct X { };
    struct Y { };
    BOOST_HANA_STATIC_ASSERT(instantiates<Comparable, X, Y>);
    BOOST_HANA_STATIC_ASSERT(!equal(X{}, Y{}));

    // No instance is provided when the two objects are of the same data type.
    BOOST_HANA_STATIC_ASSERT(!instantiates<Comparable, X, X>);
    BOOST_HANA_STATIC_ASSERT(!instantiates<Comparable, Y, Y>);
}