zeus

0 #include <utility>

1 #include <functional>

2 #include <string>

3

4 template<class Member, class Value>

5 struct update {

6 update(Member member, Value value) : member(member), value(value) {}

7

8 template<typename Record>

9 bool operator()(Record& record) {

10 if constexpr(std::is_invocable_r<bool, Member, Record, Value>()) {

11 return std::invoke(member, record, value);

12 } else if constexpr (std::is_invocable<Member, Record, Value>()) {

13 std::invoke(member, record, value);

14 return true;

15 } else {

16 std::invoke(member, record) = value;

17 return true;

18 }

19 }

20

21 Member member;

22 Value value;

23 };

24

25 template<class> struct is_update : std::false_type {};

26 template<class Member, class Value> struct is_update<update<Member, Value>> : std::true_type {};

27 template<class T> concept Update = is_update<T>::value;

28

29 template<class Member>

30 struct field {

31 field(Member member) : member(member) {}

32

33 template <typename Value>

34 update<Member, Value> operator=(Value value) const {

35 return {member, value};

36 }

37

38 Member member;

39 };

40

41 template<class Record>

42 class transaction {

43 Record& m_record;

44

45 bool all(auto... results) const {

46 return (... && results);

47 }

48

49 public:

50 transaction(Record& record) : m_record(record) {}

51

52 bool operator()(Update auto... updates) {

53 auto tmp = m_record;

54

55 if(all(updates(tmp)...)) {

56 std::swap(m_record, tmp);

57 return true;

58 }

59

60 return false;

61 }

62 };

63

64 template<typename Record>

65 auto with(Record& record) {

66 return transaction(record);

67 }

68

69 #include <iostream>

70

71 int main() {

72 struct person {

73 std::string name;

74 std::string surname;

75 unsigned age;

76 };

77

78 auto name = field { &person::name };

79 auto surname = field { &person::surname };

80 auto age = field { &person::age };

81

82 person martha;

83 with(martha) (

84 name = "Martha",

85 surname = "Jones",

86 age = 42

87 );

88

89 std::cout << martha.name << ' ' << martha.surname << ' ' << martha.age << std::endl;

90 }

91