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digitalmars.D.learn - "is not an lvalue" when passing template function to spawn function

reply Bienlein <fm2002 web.de> writes:
Hello,

I get the error "`addToBiz(T)(Biz!T biz)` is not an lvalue and 
cannot be modified" when compiling the code below. Can't find a 
way how to do it right. Am a D newbie and would appreciate some 
help.

Thank you, Bienlein



class Biz(T) {

         private T value;

         this(T value) {
             this.value = value;	
         }

}

     static void addToBiz(T)(Biz!T biz)
     {
         // ...
     }


     int main()
     {
         auto biz = new Biz!int(123);
         spawn(&addToBiz, biz);
     }
Nov 08 2023
parent reply Paul Backus <snarwin gmail.com> writes:
On Wednesday, 8 November 2023 at 16:30:49 UTC, Bienlein wrote:

 Hello,

 I get the error "`addToBiz(T)(Biz!T biz)` is not an lvalue and 
 cannot be modified" when compiling the code below. Can't find a 
 way how to do it right. Am a D newbie and would appreciate some 
 help.

 [...]

     static void addToBiz(T)(Biz!T biz)
     {
         // ...
     }


     int main()
     {
         auto biz = new Biz!int(123);
         spawn(&addToBiz, biz);
     }


This is a really bad error message.

The actual problem here is that you can't take the address of a 
template without instantiating it first. To make your example 
work, replace `&addToBiz` with `&addToBiz!int`, like this:

     spawn(&addToBiz!int, biz);
Nov 08 2023
parent reply Bienlein <fm2002 web.de> writes:
On Wednesday, 8 November 2023 at 16:47:02 UTC, Paul Backus wrote:

 On Wednesday, 8 November 2023 at 16:30:49 UTC, Bienlein wrote:
 ...
 The actual problem here is that you can't take the address of a 
 template without instantiating it first. To make your example 
 work, replace `&addToBiz` with `&addToBiz!int`, like this:

     spawn(&addToBiz!int, biz);


Thanks, Paul. This helped a step further. When applying your 
change it looks like this:

Biz!int biz = new Biz!int(123);
spawn(&addToBiz!int, biz);

Then I get this error: 'Error: static assert:  "Aliases to 
mutable thread-local data not allowed."'

For this error I found this in the Internet: 
https://stackoverflow.com/questions/14395018/aliases-to-mutable-thread-local-data-not-allowed

But this change, did not help:

spawn(&addToBiz!int, cast(shared) biz);

Then I moved "Biz!int biz = new Biz!int(123);" out of the main 
function. Compiler complains about static this. Okay, then the 
code outside the main function now looks this way:


class Biz(T) {

     private T value;

     this(T value) {
         this.value = value;
     }

}

static void addToBiz(T)(Biz!T biz)
{
     // ...
}


Biz!int biz;

static this() {
     biz = new Biz!int(123);
}



int main()
{
    // ...
}

However, this results in no gain as the compiler now shows the 
initial error again: 'Error: static assert:  "Aliases to mutable 
thread-local data not allowed."'

Everything I tried on my own was also to no avail. If someone 
could gould give me a hint again ... ;-)

Thank you.
Nov 09 2023
parent reply Bienlein <fm2002 web.de> writes:
On Thursday, 9 November 2023 at 09:40:47 UTC, Bienlein wrote:

 On Wednesday, 8 November 2023 at 16:47:02 UTC, Paul Backus 
 wrote:

 On Wednesday, 8 November 2023 at 16:30:49 UTC, Bienlein wrote:
 ...
 The actual problem here is that you can't take the address of 
 a template without instantiating it first. To make your 
 example work, replace `&addToBiz` with `&addToBiz!int`, like 
 this:

     spawn(&addToBiz!int, biz);


 Thanks, Paul. This helped a step further. When applying your 
 change it looks like this:

 Biz!int biz = new Biz!int(123);
 spawn(&addToBiz!int, biz);

 Then I get this error: 'Error: static assert:  "Aliases to 
 mutable thread-local data not allowed."'

 For this error I found this in the Internet: 
 https://stackoverflow.com/questions/14395018/aliases-to-mutable-thread-local-data-not-allowed

 But this change, did not help:

 spawn(&addToBiz!int, cast(shared) biz);

 Then I moved "Biz!int biz = new Biz!int(123);" out of the main 
 function. Compiler complains about static this. Okay, then the 
 code outside the main function now looks this way:


 class Biz(T) {

     private T value;

     this(T value) {
         this.value = value;
     }

 }

 static void addToBiz(T)(Biz!T biz)
 {
     // ...
 }


 Biz!int biz;

 static this() {
     biz = new Biz!int(123);
 }



 int main()
 {
    // ...
 }

 However, this results in no gain as the compiler now shows the 
 initial error again: 'Error: static assert:  "Aliases to 
 mutable thread-local data not allowed."'

 Everything I tried on my own was also to no avail. If someone 
 could gould give me a hint again ... ;-)

 Thank you.


If I supply a callback function with the parameter not being an 
instance from a parameterized class I get the same error. The 
problem seems to be that the parameter of the callback function 
takes on object as a parameter and not a built-in type like int 
or String.

The samples on how to use the spawn function on dlang.org does 
not contain a sample on how to get things to work with a objecgt 
being supllied as parameter to the callback function
Nov 09 2023
parent Bienlein <fm2002 web.de> writes:
On Thursday, 9 November 2023 at 10:14:46 UTC, Bienlein wrote:

 On Thursday, 9 November 2023 at 09:40:47 UTC, Bienlein wrote:

 On Wednesday, 8 November 2023 at 16:47:02 UTC, Paul Backus 
 wrote:

 On Wednesday, 8 November 2023 at 16:30:49 UTC, Bienlein wrote:
 ...
 The actual problem here is that you can't take the address of 
 a template without instantiating it first. To make your 
 example work, replace `&addToBiz` with `&addToBiz!int`, like 
 this:

     spawn(&addToBiz!int, biz);






All right. It seems I cannot pass on an object. So I store the 
object in a global and access it from the callback function 
passed to spawn.
Nov 09 2023