Re: Coreect way to create delegate for struct method.

[Alex via Digitalmars-d-learn]

On Tuesday, 21 August 2018 at 21:29:38 UTC, Andrey wrote:
Hello,
This is a code:
import std.stdio;
struct Test
{
   static Test opCall()
   {
       Test test;
       test.handler = &test.one;

       return test;
   }

   void one() const { writeln("In handler: Address = ", &this, 
"; Text = ", text); }

   void execute()
   {
       text = "Inited!";
       writeln("Before: Address = ", &this, "; Text = ", text);
       handler();
   }

   void delegate() const handler = void;
   string text = "NoValue";
}

struct Qwerty
{
   void prepare()
   {
       _test = Test();
   }

   void execute()
   {
       _test.execute();
   }

private:
   Test _test  = void;
}

void main()
{
   Qwerty qwerty;
   qwerty.prepare();
   qwerty.execute();
}

Here I try to make a delegate for struct "Test" and method 
"one()".
When I launch it then I get this output:
Before: Address = 7FFC096A2C20; Text = Inited!
In handler: Address = 7FFC096A2BE8; Text = NoValue

It means that my delegate captures one object of Test, but in 
place of call uses another...
I want just to save my method into variable and after that use 
it on some arbitrary object of type "Test". How to do it in D?

In C++ it is very easy:
test.handler = &Test::one;
and call:
(this->*handler)();
or
(someTestObjPtr->*handler)();
I know axactly that in the first variant a context will be 
"this", and in the second - "someTestObjPtr".

Maybe, like this:

´´´
import std.stdio;

struct Test
{
    static auto opCall()
    {
        auto test = new Test();
        test.handler = &test.one;
        return test;
    }

    void one() const { writeln("In handler: Address = ", &this, 
"; Text = ", text); }

    void execute()
    {
        text = "Inited!";
        writeln("Before: Address = ", &this, "; Text = ", text);
        handler();
    }

    void delegate() const handler = void;
    string text = "NoValue";
}

struct Qwerty
{
    void prepare()
    {
        _test = Test();
    }

    void execute()
    {
        _test.execute();
    }

private:
    Test* _test  = void;
}

void main()
{
    Qwerty qwerty;
    qwerty.prepare();
    qwerty.execute();
}
´´´