[cfe-dev] question about initializing multiple members of unions

[Matthew Curtis]

On 9/16/2013 1:05 PM, Richard Smith wrote:
 > On Mon, Sep 16, 2013 at 7:11 AM, Matthew Curtis 
<mcurtis at codeaurora.org> wrote:
 >
 >     Richard, Doug,
 >
 >     Do either of you have an recommendation here?
 >
 >     Eli recommends GCC's behavior, while Philip Reames and Gao would 
both prefer the alternate behavior below (same as a series of assignments).
 >
 > Can you explain how treating the designated initializers as a 
sequence of assignments could give the object the value {0, 1, 2, 3}?

Given the following program :

     #include <stdio.h>

     typedef union {
       struct {
         int zero;
         int one;
         int two;
         int three;
       } a;
       int b[4];
     } my_agg_t;

     // set value using initializer
     my_agg_t X = {
       .b[0] = 0,
       .a.one = 1,
       .b[2] = 2,
       .a.three = 3,
     };

     int main(int argc, char *argv[])
     {

       // set value using series of assignments
       my_agg_t Y;
       Y.b[0] = 0;
       Y.a.one = 1;
       Y.b[2] = 2;
       Y.a.three = 3;

       printf("X:%d,%d,%d,%d\n", X.a.zero, X.a.one, X.a.two, X.a.three);
       printf("Y:%d,%d,%d,%d\n", Y.a.zero, Y.a.one, Y.a.two, Y.a.three);
     }

GCC Produces:

     X:0,0,0,3
     Y:0,1,2,3

I believe Goa and Philip Reames are arguing that it seems very 
unintuitive that while the source code for setting X and setting Y look 
very similar they produce very different results.

 > I'm reasonably convinced that this is a hole in the C standard -- it 
doesn't specify what to do in this case -- and Eli's approach seems 
completely reasonable to me. Plus it has the benefit of > being 
compatible with at least two other major compilers (g++ and EDG), and 
doesn't require us to invent a new representation for initializer lists.

On 9/16/2013 5:28 PM, Richard Smith wrote:
 > I still think Eli's approach is the right one here.

I agree.

Does this require further discussion or can I submit a patch that 
implements this behavior?

Thanks,
Matthew

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