boost::hana::map< Pairs > Struct Template Reference

Description

template<typename ... Pairs>
struct boost::hana::map< Pairs >

Basic associative container requiring unique, Comparable and Hashable keys.

The order of the elements of the map is unspecified. Also, all the keys must be Hashable, and any two keys with equal hashes must be Comparable with each other at compile-time.

Note

The actual representation of a hana::map is an implementation detail. As such, one should not assume anything more than what is explicitly documented as being part of the interface of a map, such as:

• the presence of additional constructors
• the presence of additional assignment operators
• the fact that hana::map<Pairs...> is, or is not, a dependent type

In particular, the last point is very important; hana::map<Pairs...> is basically equivalent to

decltype(hana::make_pair(std::declval<Pairs>()...))

which is not something that can be pattern-matched on during template argument deduction, for example. More details in the tutorial.

Modeled concepts

1. Comparable
   Two maps are equal iff all their keys are equal and are associated to equal values.

   // 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 <boost/hana/assert.hpp>
   #include <boost/hana/integral_constant.hpp>
   #include <boost/hana/map.hpp>
   #include <boost/hana/pair.hpp>
   #include <boost/hana/type.hpp>
    
   #include <string>
   namespace hana = boost::hana;
   using namespace std::literals;
    
    
   int main() {
       BOOST_HANA_RUNTIME_CHECK(
           hana::make_map(
               hana::make_pair(hana::char_c<'a'>, "foobar"s),
               hana::make_pair(hana::type_c<int&&>, nullptr)
           )
           ==
           hana::make_map(
               hana::make_pair(hana::type_c<int&&>, (void*)0),
               hana::make_pair(hana::char_c<'a'>, "foobar"s)
           )
       );
   }

2. Searchable
   A map can be searched by its keys with a predicate yielding a compile-time Logical. Also note that operator[] can be used instead of at_key.

   // 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 <boost/hana/assert.hpp>
   #include <boost/hana/at_key.hpp>
   #include <boost/hana/equal.hpp>
   #include <boost/hana/find.hpp>
   #include <boost/hana/integral_constant.hpp>
   #include <boost/hana/map.hpp>
   #include <boost/hana/optional.hpp>
   #include <boost/hana/pair.hpp>
   #include <boost/hana/type.hpp>
   namespace hana = boost::hana;
    
    
   constexpr auto m = hana::make_map(
       hana::make_pair(hana::type_c<int>, 'i'),
       hana::make_pair(hana::type_c<float>, 'f')
   );
   static_assert(hana::find(m, hana::type_c<int>) == hana::just('i'), "");
   static_assert(hana::find(m, hana::type_c<float>) == hana::just('f'), "");
   BOOST_HANA_CONSTANT_CHECK(hana::find(m, hana::type_c<void>) == hana::nothing);
   BOOST_HANA_CONSTANT_CHECK(hana::find(m, hana::int_c<3>) == hana::nothing);
    
   // operator[] is equivalent to at_key
   static_assert(m[hana::type_c<int>] == 'i', "");
   static_assert(m[hana::type_c<float>] == 'f', "");
    
   int main() { }

3. Foldable
   Folding a map is equivalent to folding a list of the key/value pairs it contains. In particular, since that list is not guaranteed to be in any specific order, folding a map with an operation that is not both commutative and associative will yield non-deterministic behavior.

   // 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 <boost/hana/assert.hpp>
   #include <boost/hana/first.hpp>
   #include <boost/hana/fold_left.hpp>
   #include <boost/hana/insert.hpp>
   #include <boost/hana/integral_constant.hpp>
   #include <boost/hana/map.hpp>
   #include <boost/hana/pair.hpp>
   #include <boost/hana/second.hpp>
   #include <boost/hana/type.hpp>
   namespace hana = boost::hana;
    
    
   int main() {
       // Given a map of (key, value) pairs, returns a map of (value, key) pairs.
       // This requires both the keys and the values to be compile-time Comparable.
       auto invert = [](auto map) {
           return hana::fold_left(map, hana::make_map(), [](auto map, auto pair) {
               return hana::insert(map, hana::make_pair(hana::second(pair), hana::first(pair)));
           });
       };
    
       auto m = hana::make_map(
           hana::make_pair(hana::type_c<int>, hana::int_c<1>),
           hana::make_pair(hana::type_c<float>, hana::int_c<2>),
           hana::make_pair(hana::int_c<3>, hana::type_c<void>)
       );
    
       BOOST_HANA_CONSTANT_CHECK(invert(m) ==
           hana::make_map(
               hana::make_pair(hana::int_c<1>, hana::type_c<int>),
               hana::make_pair(hana::int_c<2>, hana::type_c<float>),
               hana::make_pair(hana::type_c<void>, hana::int_c<3>)
           )
       );
   }

Conversion from any <tt>Foldable</tt>

Any Foldable of Products can be converted to a hana::map with hana::to<hana::map_tag> or, equivalently, hana::to_map. If the Foldable contains duplicate keys, only the value associated to the first occurence of each key is kept.

// 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 <boost/hana/assert.hpp>
#include <boost/hana/core/to.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    auto xs = hana::make_tuple(
        hana::make_pair(hana::type_c<int>, "abcd"s),
        hana::make_pair(hana::type_c<void>, 1234),
        hana::make_pair(hana::type_c<int>, nullptr)
    );
 
    BOOST_HANA_RUNTIME_CHECK(
        hana::to<hana::map_tag>(xs) == hana::make_map(
            hana::make_pair(hana::type_c<int>, "abcd"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
    );
}

Example

// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
 
#include <boost/hana/assert.hpp>
#include <boost/hana/at_key.hpp>
#include <boost/hana/contains.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/string.hpp>
 
#include <functional>
#include <string>
#include <vector>
namespace hana = boost::hana;
 
 
template <typename ...Events>
struct event_system {
    using Callback = std::function<void()>;
    hana::map<hana::pair<Events, std::vector<Callback>>...> map_;
 
    template <typename Event, typename F>
    void on(Event e, F handler) {
        auto is_known_event = hana::contains(map_, e);
        static_assert(is_known_event,
            "trying to add a handler to an unknown event");
 
        map_[e].push_back(handler);
    }
 
    template <typename Event>
    void trigger(Event e) const {
        auto is_known_event = hana::contains(map_, e);
        static_assert(is_known_event,
            "trying to trigger an unknown event");
 
        for (auto& handler : this->map_[e])
            handler();
    }
};
 
template <typename ...Events>
event_system<Events...> make_event_system(Events ...events) {
    return {};
}
 
 
int main() {
    auto events = make_event_system(
        BOOST_HANA_STRING("foo"),
        BOOST_HANA_STRING("bar"),
        BOOST_HANA_STRING("baz")
    );
 
    std::vector<std::string> triggered_events;
    events.on(BOOST_HANA_STRING("foo"), [&] {
        triggered_events.push_back("foo:1");
    });
    events.on(BOOST_HANA_STRING("foo"), [&] {
        triggered_events.push_back("foo:2");
    });
    events.on(BOOST_HANA_STRING("bar"), [&] {
        triggered_events.push_back("bar:1");
    });
    events.on(BOOST_HANA_STRING("baz"), [&] {
        triggered_events.push_back("baz:1");
    });
 
    events.trigger(BOOST_HANA_STRING("foo"));
    events.trigger(BOOST_HANA_STRING("bar"));
    events.trigger(BOOST_HANA_STRING("baz"));
 
    BOOST_HANA_RUNTIME_CHECK(triggered_events == std::vector<std::string>{
        "foo:1", "foo:2", "bar:1", "baz:1"
    });
}

Synopsis of associated functions
template<>
constexpr auto	make< map_tag >
	Function object for creating a hana::map. More...
constexpr auto	make_map = make<map_tag>
	Alias to make<map_tag>; provided for convenience. More...
constexpr auto	to_map = to<map_tag>
	Equivalent to to<map_tag>; provided for convenience.
constexpr auto	keys
	Returns a Sequence of the keys of the map, in unspecified order. More...
constexpr auto	values
	Returns a Sequence of the values of the map, in unspecified order. More...
constexpr auto	insert
	Inserts a new key/value pair in a map. More...
constexpr auto	erase_key
	Removes a key/value pair from a map. More...
constexpr auto	union_
	Returns the union of two maps. More...
constexpr auto	intersection
	Returns the intersection of two maps. More...
constexpr auto	difference
	Returns the difference of two maps. More...
constexpr auto	symmetric_difference
	Returns the symmetric set-theoretic difference of two maps. More...

Friends
template<typename X , typename Y >
constexpr friend auto	operator== (X &&x, Y &&y)
	Equivalent to hana::equal
template<typename X , typename Y >
constexpr friend auto	operator!= (X &&x, Y &&y)
	Equivalent to hana::not_equal

Public Member Functions
constexpr	map ()=default
	Default-construct a map. This constructor only exists when all the elements of the map are default-constructible.
constexpr	map (map const &other)=default
	Copy-construct a map from another map. This constructor only exists when all the elements of the map are copy-constructible.
constexpr	map (map &&other)=default
	Move-construct a map from another map. This constructor only exists when all the elements of the map are move-constructible.
template<typename ... P>
constexpr	map (P &&...pairs)
	Construct the map from the provided pairs. P... must be pairs of the same type (modulo ref and cv-qualifiers), and in the same order, as those appearing in Pairs.... The pairs provided to this constructor are emplaced into the map's storage using perfect forwarding.
constexpr map &	operator= (map const &other)
	Assign a map to another map with the exact same type. Only exists when all the elements of the map are copy-assignable.
constexpr map &	operator= (map &&other)
	Move-assign a map to another map with the exact same type. Only exists when all the elements of the map are move-assignable.
template<typename Key >
constexpr decltype(auto)	operator[] (Key &&key)
	Equivalent to hana::at_key

Associated functions

make< map_tag >

template<typename ... Pairs>

template<>

constexpr auto make< map_tag >

related

Initial value:

= [](auto&& ...pairs) {
        return map<implementation_defined>{forwarded(pairs)...};
    }

Function object for creating a hana::map.

Given zero or more Products representing key/value associations, make<map_tag> returns a hana::map associating these keys to these values.

make<map_tag> requires all the keys to be unique and to have different hashes. If you need to create a map with duplicate keys or with keys whose hashes might collide, use hana::to_map or insert (key, value) pairs to an empty map successively. However, be aware that doing so will be much more compile-time intensive than using make<map_tag>, because the uniqueness of keys will have to be enforced.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/core/make.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    BOOST_HANA_RUNTIME_CHECK(
        hana::make_map(
            hana::make_pair(hana::int_c<1>, "foobar"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
        ==
        hana::make<hana::map_tag>(
            hana::make_pair(hana::int_c<1>, "foobar"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
    );
}

make_map

template<typename ... Pairs>

constexpr auto make_map = make<map_tag>

related

Alias to make<map_tag>; provided for convenience.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/core/make.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    BOOST_HANA_RUNTIME_CHECK(
        hana::make_map(
            hana::make_pair(hana::int_c<1>, "foobar"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
        ==
        hana::make<hana::map_tag>(
            hana::make_pair(hana::int_c<1>, "foobar"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
    );
}

keys

template<typename ... Pairs>

constexpr auto keys

related

Initial value:

= [](auto&& map) {
        return implementation_defined;
    }

Returns a Sequence of the keys of the map, in unspecified order.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/contains.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/keys.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/permutations.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    auto m = hana::make_map(
        hana::make_pair(hana::int_c<1>, "foobar"s),
        hana::make_pair(hana::type_c<void>, 1234)
    );
 
    // The order of the keys is unspecified.
    BOOST_HANA_CONSTANT_CHECK(
        hana::keys(m) ^hana::in^ hana::permutations(hana::make_tuple(hana::int_c<1>, hana::type_c<void>))
    );
}

values

template<typename ... Pairs>

constexpr auto values

related

Initial value:

= [](auto&& map) -> decltype(auto) {
        return implementation_defined;
    }

Returns a Sequence of the values of the map, in unspecified order.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/contains.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/permutations.hpp>
#include <boost/hana/tuple.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    auto m = hana::make_map(
        hana::make_pair(hana::int_c<1>, "foobar"s),
        hana::make_pair(hana::type_c<void>, 1234)
    );
 
    // The order of the values is unspecified.
    BOOST_HANA_RUNTIME_CHECK(
        hana::values(m) ^hana::in^ hana::permutations(hana::make_tuple("foobar"s, 1234))
    );
}

insert

template<typename ... Pairs>

constexpr auto insert

related

Initial value:

= [](auto&& map, auto&& pair) {
        return tag-dispatched;
    }

Inserts a new key/value pair in a map.

Given a (key, value) pair, insert inserts this new pair into a map. If the map already contains this key, nothing is done and the map is returned as-is.

Parameters

map	The map in which to insert a (key,value) pair.
pair	An arbitrary Product representing a (key, value) pair to insert in the map. The key must be compile-time Comparable.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/insert.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    auto m = hana::make_map(
        hana::make_pair(hana::type_c<int>, "abcd"s),
        hana::make_pair(hana::type_c<void>, 1234)
    );
 
    BOOST_HANA_RUNTIME_CHECK(
        hana::insert(m, hana::make_pair(hana::type_c<float>, 'x')) ==
        hana::make_map(
            hana::make_pair(hana::type_c<int>, "abcd"s),
            hana::make_pair(hana::type_c<void>, 1234),
            hana::make_pair(hana::type_c<float>, 'x')
        )
    );
 
    BOOST_HANA_RUNTIME_CHECK(hana::insert(m, hana::make_pair(hana::type_c<void>, 'x')) == m);
}

erase_key

template<typename ... Pairs>

constexpr auto erase_key

related

Initial value:

= [](auto&& map, auto&& key) {
        return tag-dispatched;
    }

Removes a key/value pair from a map.

Returns a new hana::map containing all the elements of the original, except for the (key, value) pair whose key compares equal to the given key. If the map does not contain such an element, a new map equal to the original is returned.

Parameters

map	The map in which to erase a key.
key	A key to remove from the map. It must be compile-time Comparable.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/erase_key.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/string.hpp>
#include <boost/hana/type.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace std::literals;
 
 
int main() {
    auto m = hana::make_map(
        hana::make_pair(hana::type_c<int>, "abcd"s),
        hana::make_pair(hana::type_c<void>, 1234),
        hana::make_pair(BOOST_HANA_STRING("foobar!"), hana::type_c<char>)
    );
 
    BOOST_HANA_RUNTIME_CHECK(
        hana::erase_key(m, BOOST_HANA_STRING("foobar!")) ==
        hana::make_map(
            hana::make_pair(hana::type_c<int>, "abcd"s),
            hana::make_pair(hana::type_c<void>, 1234)
        )
    );
 
    BOOST_HANA_RUNTIME_CHECK(hana::erase_key(m, hana::type_c<char>) == m);
}

union_

template<typename ... Pairs>

constexpr auto union_

related

Initial value:

= [](auto&& xs, auto&& ys) {
        return tag-dispatched;
    }

Returns the union of two maps.

Given two maps xs and ys, hana::union_(xs, ys) is a new map containing all the elements of xs and all the elements of ys, without duplicates. If both xs and ys contain an element with the same key, the one in ys is taken. Functionally, hana::union_(xs, ys) is equivalent to

hana::fold_left(xs, ys, hana::insert)

Parameters

xs,ys

The two maps to compute the union of.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/string.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/type.hpp>
#include <boost/hana/union.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
 
 
static auto m1 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key1"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key2"), hana::type_c<std::string>)
);
 
static auto m2 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key3"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key4"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key5"), hana::type_c<std::string>)
);
 
BOOST_HANA_CONSTANT_CHECK(hana::union_(m1, m2) == hana::make_map(
       hana::make_pair(BOOST_HANA_STRING("key1"), hana::type_c<std::string>),
       hana::make_pair(BOOST_HANA_STRING("key2"), hana::type_c<std::string>),
       hana::make_pair(BOOST_HANA_STRING("key3"), hana::type_c<std::string>),
       hana::make_pair(BOOST_HANA_STRING("key4"), hana::type_c<std::string>),
       hana::make_pair(BOOST_HANA_STRING("key5"), hana::type_c<std::string>)
));
 
constexpr auto m3 = hana::make_map(
    hana::make_pair(hana::type_c<int>, hana::int_c<1>),
    hana::make_pair(hana::type_c<bool>, hana::bool_c<true>)
);
 
constexpr auto m4 = hana::make_map(
       hana::make_pair(hana::type_c<char>, hana::char_c<'c'>),
       hana::make_pair(hana::type_c<bool>, hana::bool_c<false>)
);
 
BOOST_HANA_CONSTANT_CHECK(hana::union_(m3, m4) == hana::make_map(
       hana::make_pair(hana::type_c<int>, hana::int_c<1>),
       hana::make_pair(hana::type_c<bool>, hana::bool_c<false>),
       hana::make_pair(hana::type_c<char>, hana::char_c<'c'>)
));
 
int main() { }

intersection

template<typename ... Pairs>

constexpr auto intersection

related

Initial value:

= [](auto&& xs, auto&& ys) {
        return tag-dispatched;
    }

Returns the intersection of two maps.

Given two maps xs and ys, intersection(xs, ys) is a new map containing exactly those (key, value) pairs from xs, for which key is present in ys. In other words, the following holds for any object pair(k, v):

pair(k, v) ^in^ intersection(xs, ys) if and only if (k, v) ^in^ xs && k ^in^ keys(ys)

Note

This function is not commutative, i.e. intersection(xs, ys) is not necessarily the same as intersection(ys, xs). Indeed, the set of keys in intersection(xs, ys) is always the same as the set of keys in intersection(ys, xs), but the value associated to each key may be different. intersection(xs, ys) contains values present in xs, and intersection(ys, xs) contains values present in ys.

Parameters

xs,ys

Two maps to intersect.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/intersection.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/string.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
 
 
static auto m1 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key1"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key2"), hana::type_c<std::string>)
);
 
static auto m2 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key3"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key4"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key5"), hana::type_c<std::string>)
);
 
BOOST_HANA_CONSTANT_CHECK(hana::intersection(m1, m2) == hana::make_map());
 
static auto m3 = hana::make_map(
    hana::make_pair(hana::type_c<int>, hana::int_c<1>),
    hana::make_pair(hana::type_c<bool>, hana::bool_c<true>),
    hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("hana")),
    hana::make_pair(hana::type_c<float>, hana::int_c<100>)
);
 
static auto m4 = hana::make_map(
       hana::make_pair(hana::type_c<char>, hana::char_c<'c'>),
       hana::make_pair(hana::type_c<bool>, hana::bool_c<false>),
       hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("boost"))
);
 
BOOST_HANA_CONSTANT_CHECK(hana::intersection(m3, m4) == hana::make_map(
    hana::make_pair(hana::type_c<bool>, hana::bool_c<true>),
    hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("hana"))
));
 
int main() { }

difference

template<typename ... Pairs>

constexpr auto difference

related

Initial value:

= [](auto&& xs, auto&& ys) {
        return tag-dispatched;
    }

Returns the difference of two maps.

Given two maps xs and ys, difference(xs, ys) is a new map containing exactly those (key, value) pairs from xs, for which key is not present in keys(ys). In other words, the following holds for any object pair(k, v):

pair(k, v) ^in^ difference(xs, ys) if and only if (k, v) ^in^ xs && k ^not in^ keys(ys)

Note

This function is not commutative, i.e. difference(xs, ys) is not necessarily the same as difference(ys, xs). Indeed, consider the case where xs is empty and ys isn't. In that case, difference(xs, ys) is empty, but difference(ys, xs) is equal to ys. For symmetric version of this operation, see symmetric_difference.

Parameters

xs,ys

Two maps to compute the difference of.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/intersection.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/string.hpp>
 
#include <string>
namespace hana = boost::hana;
using namespace hana::literals;
 
 
static auto m1 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key1"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key2"), hana::type_c<std::string>)
);
 
static auto m2 = hana::make_map(
    hana::make_pair(BOOST_HANA_STRING("key3"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key4"), hana::type_c<std::string>),
    hana::make_pair(BOOST_HANA_STRING("key5"), hana::type_c<std::string>)
);
 
BOOST_HANA_CONSTANT_CHECK(hana::intersection(m1, m2) == hana::make_map());
 
static auto m3 = hana::make_map(
    hana::make_pair(hana::type_c<int>, hana::int_c<1>),
    hana::make_pair(hana::type_c<bool>, hana::bool_c<true>),
    hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("hana")),
    hana::make_pair(hana::type_c<float>, hana::int_c<100>)
);
 
static auto m4 = hana::make_map(
       hana::make_pair(hana::type_c<char>, hana::char_c<'c'>),
       hana::make_pair(hana::type_c<bool>, hana::bool_c<false>),
       hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("boost"))
);
 
BOOST_HANA_CONSTANT_CHECK(hana::intersection(m3, m4) == hana::make_map(
    hana::make_pair(hana::type_c<bool>, hana::bool_c<true>),
    hana::make_pair(hana::type_c<std::string>, BOOST_HANA_STRING("hana"))
));
 
int main() { }

symmetric_difference

template<typename ... Pairs>

constexpr auto symmetric_difference

related

Initial value:

= [](auto&& xs, auto&& ys) {
        return tag-dispatched;
    }

Returns the symmetric set-theoretic difference of two maps.

Given two sets xs and ys, symmetric_difference(xs, ys) is a new map containing all the elements of xs whose keys are not contained in keys(ys), and all the elements of ys whose keys are not contained in keys(xs). The symmetric difference of two maps satisfies the following:

symmetric_difference(xs, ys) == union_(difference(xs, ys), difference(ys, xs))

Parameters

xs,ys

Two maps to compute the symmetric difference of.

Example

// 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 <boost/hana/assert.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/symmetric_difference.hpp>
 
namespace hana = boost::hana;
 
 
constexpr auto m1 = hana::make_map(
    hana::make_pair(hana::type_c<int>, 1),
    hana::make_pair(hana::type_c<bool>, hana::true_c)
);
 
constexpr auto m2 = hana::make_map(
    hana::make_pair(hana::type_c<float>, 1.0),
    hana::make_pair(hana::type_c<long long>, 2LL),
    hana::make_pair(hana::type_c<int>, 3)
);
 
constexpr auto result_m = hana::make_map(
    hana::make_pair(hana::type_c<bool>, hana::true_c),
    hana::make_pair(hana::type_c<float>, 1.0),
    hana::make_pair(hana::type_c<long long>, 2LL)
);
 
int main() {
    BOOST_HANA_RUNTIME_CHECK(
            hana::symmetric_difference(m1, m2) == result_m
    );
}