如何连接两个 Boost Hana 地图？

Question

我有两个 boost::hana::map 要连在一起。

constexpr auto m1 = hana::make_map( 
    hana::make_pair("key1"_s, hana::type_c<std::string>), 
    hana::make_pair("key2"_s, hana::type_c<std::string>) 
); 

constexpr auto m2 = hana::make_map( 
    hana::make_pair("key3"_s, hana::type_c<std::string>), 
    hana::make_pair("key4"_s, hana::type_c<std::string>), 
    hana::make_pair("key5"_s, hana::type_c<std::string>) 
);

如何获取包含两者的地图，如下所示？

constexpr auto result = hana::make_map( 
    hana::make_pair("key1"_s, hana::type_c<std::string>), 
    hana::make_pair("key2"_s, hana::type_c<std::string>), 
    hana::make_pair("key3"_s, hana::type_c<std::string>), 
    hana::make_pair("key4"_s, hana::type_c<std::string>), 
    hana::make_pair("key5"_s, hana::type_c<std::string>) 
);

Answer 1

较新版本

^{(Boost.Hana > 1.2, 提升 >= 1.65)}

有boost::hana::union_函数可以加入boost::hana::maps。

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.

Cite from Boost.Hana docs

还有例子：

// Simple example
constexpr auto m1 = hana::make_map(
    hana::make_pair("key1"_s, hana::type_c<std::string>),
    hana::make_pair("key2"_s, hana::type_c<std::string>)
);
constexpr auto m2 = hana::make_map(
    hana::make_pair("key3"_s, hana::type_c<std::string>),
    hana::make_pair("key4"_s, hana::type_c<std::string>),
    hana::make_pair("key5"_s, hana::type_c<std::string>)
);
BOOST_HANA_CONSTANT_CHECK(hana::union_(m1, m2) == hana::make_map(
       hana::make_pair("key1"_s, hana::type_c<std::string>),
       hana::make_pair("key2"_s, hana::type_c<std::string>),
       hana::make_pair("key3"_s, hana::type_c<std::string>),
       hana::make_pair("key4"_s, hana::type_c<std::string>),
       hana::make_pair("key5"_s, hana::type_c<std::string>)
));

// Example with overwriting pair (the same key)
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'>)
));

完整的交互式示例here on Godbolt在线编译器。

旧版本

^{(Boost.Hana <= 1.1, 提升 <= 1.64)}

基于 some Boost forums topic 的修改示例。

constexpr auto map1 = hana::make_map( 
    hana::make_pair(hana::type_c<TwoOtherKeys1>,      hana::type_c<SomeValue>),
    hana::make_pair(hana::type_c<SameKey_SameType>,   hana::type_c<SomeValue>),
    hana::make_pair(hana::type_c<SameKey_DifferType>, hana::type_c<SomeValue>)
);
constexpr auto map2 = hana::make_map( 
    hana::make_pair(hana::type_c<TwoOtherKeys2>,      hana::type_c<SomeValue>), 
    hana::make_pair(hana::type_c<SameKey_SameType>,   hana::type_c<SomeValue>), 
    hana::make_pair(hana::type_c<SameKey_DifferType>, hana::type_c<OtherValue>) 
); 

// Creates result sequence by inserting all of (source) `map1` sequence into (initial) `map1` sequence. 
// Since both sequences are maps with unique keys, keys that exist in both source and initial map got overwrited.
constexpr auto result = hana::fold(map1, map2, hana::insert);

// Excepted result
constexpr auto expected = hana::make_map(
    // Diffrent keys pairs are copied.
    hana::make_pair(hana::type_c<TwoOtherKeys1>, hana::type_c<SomeValue>),
    hana::make_pair(hana::type_c<TwoOtherKeys2>, hana::type_c<SomeValue>),

    // The same key and type pairs are just passed.
    hana::make_pair(hana::type_c<SameKey_SameType>, hana::type_c<SomeValue>),   

    // The same key, differ type - overwrited.
    hana::make_pair(hana::type_c<SameKey_DifferType>, hana::type_c<OtherValue>)
);

// Assertion to confirm exceptation
static_assert(result == expected, ""); // OK

完整的交互示例here on Godbolt在线编译器。

该解决方案与最新版本 Boost.Hana 中的解决方案完全相同。

解决 Louis Dionne 的所有学分。

如何连接两个 Boost Hana 地图？

How to join two Boost Hana maps?

c++

boost

metaprogramming

boost-hana

较新版本

旧版本