How to get the pointer of "this" ?
bauss
jj_1337 at live.dk
Tue May 26 22:23:19 UTC 2020
On Tuesday, 26 May 2020 at 12:08:29 UTC, Johannes Loher wrote:
> 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.
You can just do this to get around it:
auto button = this;
log(&button);
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