c++ - 如何连接两个 Boost Hana map ？

Question

我有两个 boost::hana::map 要连接在一起。

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 map ，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

更新版本

^{(Boost.Hana > 1.2，Boost >= 1.65)}

boost::hana::union_函数用于连接boost::hana::map

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'>)
));

完整的交互示例 here on Godbolt在线编译器。

---

旧版本

^{(Boost.Hana <= 1.1，Boost <= 1.64)}

基于 some Boost forums topic 中的修改示例.

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的 Boost.Hana 中的解决方案完全相同。

---

Louis Dionne 解决方案的所有功劳.