How to get the pointer of "this" ?

[Johannes Loher]

On Tuesday, 26 May 2020 at 11:44:58 UTC, Vinod K Chandran wrote:
> On Monday, 25 May 2020 at 16:39:30 UTC, Mike Parker wrote:
>> On Monday, 25 May 2020 at 08:39:23 UTC, John Burton wrote:
>>
>>> I believe that in D *this* is a reference to the
>>> object and not a pointer like in C++.
>>> So I think that writing &this might be what you need?
>>
>> No. A class reference is a pointer under the hood. Getting its 
>> address will result in a pointer to the reference variable 
>> itself, not to the class instance. When passing a reference to 
>> a C API, casting it directly to the C type is correct.
>
> Try this code. This will reproduce the same error.
> import std.stdio : log = writeln;

> void main() {
>      log("Let's check whether 'this' is an lvalue or not.");
>      Button btn = new Button("A button");
> }
>
> class Button {
>     this(string btntext)    {
>         mtext = btntext;
>         log("button created with the name , ", btntext);
>         log(&this);
>     }
>     private:
>     string mt
> }
It doesn't compile, the line

string mt

should be

string mtext;

instead. Indeed, we get a compiler error:

Error: this is not an lvalue and cannot be modified.

The problem is in line 11: You are trying to get the address of 
`this`. But `this` is an lvalue, so it does not have an address 
you could take. It becomes mir clear that this doesn’t work if 
you consider other lvalues, like literals:

int* = &1; // doesn’t compile, can’t take the address of an 
lvalue.

In this code example, the correct thing to do is to simply not 
take the address but pass `this` to `writeln`. That compiles and 
results in the following output:

Let's check whether 'this' is an lvalue or not.
button created with the name , A button
onlineapp.Button

(The module is onlineapp because I ran it in run.dlang.io)

I don't know what the correct thing would be in your original 
code though.