Re: union initalization

[Johannes Pfau via Digitalmars-d-learn]

Am Fri, 22 Jul 2016 01:48:52 +
schrieb Rufus Smith :

> I would like to combine two types
> 
> 
> template Foo(A, B = 4)
> {
>  union
>  {
> byte b = B;
> int a = A << 8;
>  }
> }
> 
> 
> I get an error about overlapping default initialization. They 
> don't actually overlap in this case because of the shift.

To be pedantic about this: The initializers actually do overlap. As A
<< 8 shifts in zeroes, these bits are initialized as well. I think
there's no idiomatic way to initialize only part of an integer.

Re: 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

Re: union initalization

[Rufus Smith via Digitalmars-d-learn]

On Friday, 22 July 2016 at 02:08:06 UTC, Ali Çehreli wrote:

On 07/21/2016 06:48 PM, Rufus Smith wrote:
> I would like to combine two types
>
>
> template Foo(A, B = 4)
> {
>  union
>  {
> byte b = B;
> int a = A << 8;
>  }
> }
>
> This packs a and b together in to an int bytes, saving an
int(rather
> than storing a and b in an int each) and makes it easier to
access.

Bitfields may actually be useful in this case:

  https://dlang.org/phobos/std_bitmanip.html#.bitfields



They don't allow default assignment though?


> I get an error about overlapping default initialization. They
don't
> actually overlap in this case because of the shift.

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. Bitfields seem like it would 
be the best way but I require default initialization. I want to 
hide as details of the work on unpacking as possible.

Re: union initalization

[Ali Çehreli via Digitalmars-d-learn]

On 07/21/2016 06:48 PM, Rufus Smith wrote:
> I would like to combine two types
>
>
> template Foo(A, B = 4)
> {
>  union
>  {
> byte b = B;
> int a = A << 8;
>  }
> }
>
> This packs a and b together in to an int bytes, saving an int(rather
> than storing a and b in an int each) and makes it easier to access.

Bitfields may actually be useful in this case:

  https://dlang.org/phobos/std_bitmanip.html#.bitfields

> I get an error about overlapping default initialization. They don't
> actually overlap in this case because of the shift.

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

union initalization

[Rufus Smith via Digitalmars-d-learn]

I would like to combine two types


template Foo(A, B = 4)
{
union
{
   byte b = B;
   int a = A << 8;
}
}

This packs a and b together in to an int bytes, saving an 
int(rather than storing a and b in an int each) and makes it 
easier to access.


I get an error about overlapping default initialization. They 
don't actually overlap in this case because of the shift.