union initalization

[Ali Çehreli via Digitalmars-d-learn]

On 07/21/2016 08:00 PM, Rufus Smith wrote:

 >> Bitfields may actually be useful in this case:
 >>
 >>   https://dlang.org/phobos/std_bitmanip.html#.bitfields
 >>
 >
 > They don't allow default assignment though?

A factory function can help:

import std.bitmanip;

struct S(int a_init, byte b_init) {
     mixin (bitfields!(int, "a", 24,
                       byte, "b", 8));
     @disable this();
}

auto makeS(int a_init, byte b_init = 4)() {
     auto s = S!(a_init, b_init).init;
     s.a = a_init;
     s.b = b_init;
     return s;
}

void main() {
     const s = makeS!42();
     assert(s.a == 42);
     assert(s.b == 4);
}

 >> The following at least compiles:
 >>
 >> struct Foo(int A, byte B = 4)
 >> {
 >>     union
 >>     {
 >>         byte b = void;
 >>         int a = void;
 >>         uint __both = (A << 8) | B;
 >>     }
 >> }
 >>
 >> void main() {
 >>     auto f = Foo!(42)();
 >> }
 >>
 >> Ali
 >
 > Maybe... I don't like extra member.

Me neither. Besides, 'b' coincides with a different part of 'a' 
depending on the endianness of the system.

Ali