designated_initializers.cpp

// https://godbolt.org/z/s9GheEWav
// MIT License - Copyright(c) 2023 James Edward Anhalt III - https://github.com/jeaiii

#define CPP17 0
#define CPP14 0

#if CPP17
#define AUTO_T auto
#else
#define AUTO_T unsigned long long
#endif

template<AUTO_T M, class T = int> struct arg
{
    static constexpr auto m = M;
    T n;

    template<class U> arg<m, U const&> operator=(U const& u) const { return { u }; }
};

#if CPP14
template<AUTO_T M, class U> constexpr auto as_member_p = 0;
#else
template<AUTO_T M, class U> struct as_member_ptr;
#endif

template<class U, class...Ts> U construct(Ts...ts)
{
    U u{ };
#if CPP17    
    ((u.*as_member_p<ts.m, U> = ts.n), ...);
#elif CPP14
    int _z[]{ 0, (u.*as_member_p<ts.m, U> = ts.n, 0)... }; (void)_z;
#else
    int _z[]{ 0, (u.*as_member_ptr<ts.m, U>::m = ts.n, 0)... }; (void)_z;
#endif    
    return u;
}

constexpr unsigned long long hash(char const text[], unsigned long long h = 0)
{
    return text[0] == '\0' ? h : hash(text + 1, ((h >> 5) | (h << (sizeof(h) * 8 - 5))) ^ (unsigned char)*text);
}

// do this once for all unique member names in any struct
constexpr auto $x = arg<hash("x")>{ };
constexpr auto $y = arg<hash("y")>{ };

#if CPP14
template<class U> constexpr auto as_member_p<$x.m, U> = &U::x;
template<class U> constexpr auto as_member_p<$y.m, U> = &U::y;
#else
template<class U> struct as_member_ptr<$x.m, U> { static constexpr auto m = &U::x; };
template<class U> struct as_member_ptr<$y.m, U> { static constexpr auto m = &U::y; };
#endif

struct vec2 { int x; int y; };

vec2 test()
{
    return construct<vec2>($x = 42, $y = 32);
}

vec2 test2()
{
    return construct<vec2>($y = 32, $x = 42);
}

/*
vec2 test3()
{
    return vec2{ .x = 42, .y = 32 };
}
*/

/*
MIT License
Copyright(c) 2023 James Edward Anhalt III - https://github.com/jeaiii
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/