A proposal that works like BOOST_HANA_DEFINE/ADAPT_STRUCT and makes it 5x faster.

[johnjohnlin]

Hi developers, I have used BOOST_HANA_DEFINE_STRUCT in my project and find it very useful. Thanks for the great work.
When I was using the library, I came up with an idea that makes a similar BOOST_HANA_DEFINE_STRUCT, with much shorter compile time. I made a proof-of-concept and it shows 5x time saving and 4x memory saving (will show later).

Since Hana is a library that features fast compile time, I think my proposal can help makes Hana a better library, and I want to make contribution.

The proposal is a macro like BOOST_HANA_ADAPT_STRUCT.

struct MyStruct {
    int a;
    bool b;
    string c;
    vector<int> d;
    MAKE_NAMEDTUPLE(a, b, c, d)
};
MyStruct my;

Using this macro, MyStruct almost becomes a tuple, bound with member name.

• my.get<0>() is a
• my.get<1>() is b
• my.get<2>() is c
• my.get<3>() is d
• MyStruct::get_name(0) is "a"
• MyStruct::get_name(1) is "b"
• MyStruct::get_name(2) is "c"
• MyStruct::get_name(3) is "d"
• MyStruct::num_members is 4

This acts like python namedtuple, which allows iterating all members/names using fold expression (c++17) or recursion (pre c++17).

For example, this namedtuple allowed me to create a generic argument parser works as below:

int argc;
char **argv;
// Input
argc = 14
argv = ./main.exe --a 12 --b --d [ 1 2 3 ] --c mystring --garbage extra
Argparse(my, argc, argv)
// Output
argc = 3
argv = ./main.exe --garbage extra

Or I can implement a generic JSON print function like the example in Hana::Struct.

The implemention of namedtuple is simple. With boost.preprocessor I only need 10 lines of macro, which is expanded to:

struct MyStruct {
    int a;
    bool b;
    string c;
    vector<int> d;
    // The following code can be generated by boost preprocessor
    // by MAKE_NAMEDTUPLE(a, b, c, d)
    auto& get(integral_constant<unsigned, 0>) { return a; }
    auto& get(integral_constant<unsigned, 1>) { return b; }
    auto& get(integral_constant<unsigned, 2>) { return c; }
    auto& get(integral_constant<unsigned, 3>) { return d; }
    static constexpr unsigned num_members = 4;
    const char* get_name(unsigned x) {
        static const char* names[] = {"a", "b", "c", "d", ""};
        return names[x];
    }
    template<unsigned x> auto& get() { return get(integral_constant<unsigned, x>{}); }
};

The benchmark is accessible here.
How do you think about this proposal? If you consider this is suitable, can you instruct me some ways to glue this namedtuple into Hana?

[ldionne]

Hi, and thanks for your interest!

I think the best way forward here would be to integrate with Boost.Describe as discussed in #498. WDYT?

[johnjohnlin]

Agree, seems that Boost.Describe provides sufficient functionalities for type reflection. I shall avoid inventing the wheel.

Maybe I can try whether using Boost.Describe can also provide good compile time performance.