C++: Provide constructors for auto-generated unions

[pavel-kirienko]

Instead of:

Ty_1 val;
val.foo = 123;
val.bar = baz;

go like:

const Ty_1 val(123, baz);

To facilitate immutability and URVO.

[ASMfreaK]

This partially works with brace-init, for everything except for unions.

// with C++17
const Heartbeat_1_0 hb{
  1244, // uptime
   {Health::NOMINAL},
   {Mode::WARNING},
   0x1
};

// with C++20 or extension
const Heartbeat_1_0 hb{
  .uptime=1244, // uptime
   .health={Health::NOMINAL},
   .mode={Mode::WARNING},
   .vssc=0x1
};

[pavel-kirienko]

Indeed, the members are public. I guess I am going close this and walk away with shame.

[ASMfreaK]

Unions would really need a constructor to initialize this way. We would need some kind of a tag dispatch to differentiate between different named variants.

// proposed syntax
const Value_1_0 val{Value_1_0::natural_16_tag, 1, 2, 3};

And variant construction is a problem right now. At the moment we can't constuct specific variant, we can only set value after:

Value_1_0 v;
v.set_natural_16({{10, 10, 0}});

[ASMfreaK]

I think the issue exists, just not for simple POD values as listed earlier.

[ASMfreaK]

There is also the issue of missing value init (as in the opening post)