Initialization of unions

[Graham St Jack]

Hi Justin,

I am using a struct as a discriminated union in my version of the 
concurrency framework, and it all seems to work just fine. Here is a 
code fragment from the code that implements a simple protocol. The code 
is generated by a template, so it is very easy to crank out a message 
struct with lots of different kinds of payload.


struct jobMsg {
     string name;
     this(string name) {
          this.name = name;
     }
     void read(InStream stream) {
          name = stream.get!string;
     }
     void write(OutStream stream) {
         stream(name);
     }
}
struct Message {
     uint kind;
     union {
         jobMsg job;
     }
     this(ref jobMsg msg) {
         kind = 0;
         job = msg;
     }
     this(InStream stream) {
         kind = stream.get!uint;
         switch(kind) {
             case 0: job.read(stream); break;
             default: assert(0, "Cannot read unsupported message kind");
         }
     }
     void write(OutStream stream) {
         stream(kind);
         switch(kind) {
             case 0: job.write(stream); break;
             default: assert(0, "Cannot write unsupported message kind");
         }
     }
}


On 23/09/10 21:58, Justin Johansson wrote:
> On 23/09/2010 10:14 PM, bearophile wrote:
>> Justin Johansson:
>>
>>> One of the problems with C++ is that it is not possible
>>> to create unions with non-primitive members (e.g. structs)
>>> that have constructors.
>>
>> Do you mean something like this?
>>
>> struct S1 {
>>      int y;
>>      this(int x) { y = x; }
>> }
>>
>> struct S2 {
>>      string t;
>>      this(string s) { t = s; }
>> }
>>
>> union U {
>>      S1 s1;
>>      S2 s2;
>> }
>>
>> static U u2 = { s2:S2("hello") };
>>
>> void main() {
>>      U u = U(S1(10));
>>      assert(u.s1.y == 10);
>>      u.s2 = S2("hello");
>>      assert(u.s2.t == "hello");
>>      // U u3 = U(S2("hello")); // not possible
>> }
>
> Yes, but not yes; something like that.  You are obviously
> one step ahead of me so perhaps I should give up or else
> post the exact problem in C++.  Still, it looks likes
> from what you have shown that D has some better union
> construction syntax than C++.
>
> I hope others can throw in their 2 cents.
>
> Bye,
> Justin
>


-- 
Graham St Jack