I'm making a serializer that has a variadic write method that takes arbitrary params
and serializes them;  I want to do the same thing with a read method ( pass in your
params by ref, and it populates them with data ) -  however, I run into a compiler
error - "cannot be ref" in the foreach statement...  Anyone know a workaround for this?  Is this a bug, or by design?

Simplified example:

void populateVars(T...)(T vars){
  // Generates "cannot be ref" compiler error:
  foreach(ref v; vars){
    // Populate v with some data here...
  }
}

On 12/02/2017 3:41 AM, error wrote:
> I'm making a serializer that has a variadic write method that takes
> arbitrary params
> and serializes them;  I want to do the same thing with a read method (
> pass in your
> params by ref, and it populates them with data ) -  however, I run into
> a compiler
> error - "cannot be ref" in the foreach statement...  Anyone know a
> workaround for this?  Is this a bug, or by design?
>
> Simplified example:
>
> void populateVars(T...)(T vars){
>   // Generates "cannot be ref" compiler error:
>   foreach(ref v; vars){
>     // Populate v with some data here...
>   }
> }

Try:

foreach(i, v; vars) {
	vars[i] = ...;
}

On Saturday, 11 February 2017 at 14:43:18 UTC, rikki cattermole wrote:
> Try:
>
> foreach(i, v; vars) {
> 	vars[i] = ...;
> }

Perfect! Thanks so much - I wish that hint was in the documentation for variadic functions, although I guess it suggests an inefficiency in the compiler - since there would be an additional copy of vars[i] created in v.

On Saturday, 11 February 2017 at 15:02:11 UTC, error wrote:
> On Saturday, 11 February 2017 at 14:43:18 UTC, rikki cattermole wrote:
>> Try:
>>
>> foreach(i, v; vars) {
>> 	vars[i] = ...;
>> }
>
> Perfect! Thanks so much - I wish that hint was in the documentation for variadic functions, although I guess it suggests an inefficiency in the compiler - since there would be an additional copy of vars[i] created in v.

Do you have a complete code example that gives your error? I can't reproduce it (DMD v2.073.0):

int foo(T...)(T vars) {
    int i = 0;
    foreach(ref v ; vars) {
        v = 5;
        i += v;
    }
    return i;
}
void bar(T...)(ref T vars) {
    foreach(ref v ; vars) {
        v = 3;
    }
}
void main() {
    import std.stdio;
    int x = 7;
    bar(x);
    writeln(foo(4,x,8.2)); // 15, no errors
}

On Sunday, 12 February 2017 at 03:34:19 UTC, Michael Coulombe wrote:

> Do you have a complete code example that gives your error? I can't reproduce it (DMD v2.073.0):
>
> int foo(T...)(T vars) {
>     int i = 0;
>     foreach(ref v ; vars) {
>         v = 5;
>         i += v;
>     }
>     return i;
> }
> void bar(T...)(ref T vars) {
>     foreach(ref v ; vars) {
>         v = 3;
>     }
> }
> void main() {
>     import std.stdio;
>     int x = 7;
>     bar(x);
>     writeln(foo(4,x,8.2)); // 15, no errors
> }

Oh... That's really strange;  I've updated my code to use non indexed foreach, and it's now building without the error...  I have absolutely no idea what caused the error to occur.  I've also tried reverting to earlier versions of the project, and I can't recreate the issue.  If it occurs again, I'll snapshot the project and try to make a repeatable set up.