Re: [boost] [gsoc17] Project1: Static map: Can I implement the competency test in g++-7?

Subject: Re: [boost] [gsoc17] Project1: Static map: Can I implement the competency test in g++-7?
From: Vic Luo (vicluo96_at_[hidden])
Date: 2017-03-10 15:58:30

Hi,

During the implementation of my static_map, I recently encountered a
problem: how to deal with string types?

For example:

    constexpr auto cmap_str = make_static_map({

        {"abc", 1},

        {"bcd", 2}

    });

    static_assert(cmap_str["abc"] == 1, "test");

In my implementation, KeyType requires

1. constexpr operator==, which is not provided by const char *

2. constexpr hash(const KeyType&). Since there is no such thing in
standard library, I have to implement some in this project.

Currently I would like to use string_view, since it looks like a general
solution for compile-time string with lots of constexpr functions, but I
don't know which of the following designs looks better:

Design A:

        template<typename KeyType,
        typename MappedType,
        typename EqualOp = StaticEqualOp<KeyType>,
        typename HashOp = StaticHashOp<KeyType>>
        class static_map;

        // StaticEqualOp: Specialize StaticEqualOp for those KeyTypes that

        // can construct string_view and use operator== of string_view,

        // otherwise we simply use operator== of KeyType

        // StaticHashOp: size_t operator()(const string_view&, const
string_view&)

        // for types that can construct string_view

Design B:

    We don't specialize any StaticEqualOp for const char*. Instead,
make_static_map({{"abc", 1}}) returns static_map<string_view, int>
instead of static_map<const char *, int>.

Which one is better? Or is there any more feasible design?

Thanks,

-- 
---------------------------------
Vic Luo
Shanghai Jiaotong University
Key fingerprint 0x98809ca08bf5662a