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digitalmars.D.learn - usage of ref foreach with variadic functions fails with "cannot be

reply error <error n.com> writes:
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...
   }
}
Feb 11
parent reply rikki cattermole <rikki cattermole.co.nz> writes:
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] = ...; }
Feb 11
parent reply error <error n.com> writes:
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.
Feb 11
parent reply Michael Coulombe <kirsybuu gmail.com> writes:
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 }
Feb 11
parent error <error n.com> writes:
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.
Feb 12