Address of a class object

[matheus]

On Sunday, 1 January 2023 at 09:01:24 UTC, Paul wrote:
> ...
> If the size of MyClass is 9 bytes why do MyClassO1 & O2 
> addresses only differ by 4 bytes?
>
> Because those addresses(4FFB20  4FFB24) are the addresses of 
> the class **variables**, not the addresses of the **objects** 
> themselves?

Because MyClass01 and MyClass02 are pointers and in your case 
they differ 4 bytes each other.

Now you could do this:

import std.stdio, std.traits;

class MyClass {char[16] c;}

void main() {
     writeln(" Size  Alignment  Type\n",
             "=========================");

     size_t size = __traits(classInstanceSize, MyClass);
     size_t alignment = classInstanceAlignment!MyClass;

     writefln("%4s%8s      %s",size, alignment, MyClass.stringof);

     // my test code added
     MyClass MyClassO1;
     MyClass MyClassO2;
     writeln("\n",&MyClassO1,"\t",&MyClassO2);
     writeln("\n",&(MyClassO1.c),"\t",&(MyClassO2.c));
     MyClassO1 = new MyClass();
     MyClassO2 = new MyClass();
     writeln("\n",&MyClassO1,"\t",&MyClassO2);
     writeln("\n",&(MyClassO1.c),"\t",&(MyClassO2.c));
}

In this machine it will print:

  Size  Alignment  Type
=========================
   32       8      MyClass

7FFD890C6410	7FFD890C6418 <- &MyClassO1 &MyClassO2
10	10                   <- Note here  [&(MyClassO1.c), 
&(MyClassO2.c)]
7FFD890C6410	7FFD890C6418 <- &MyClassO1 &MyClassO2 (Same address)
7FD0435D8010	7FD0435D8030 <- Now after instantiation! 
[&(MyClassO1.c), &(MyClassO2.c)]

Finally as you can see I changed your:

class MyClass {char c;}

to:

class MyClass {char[16] c;}

Because from char[1] to char[16] it will keep the address 
difference for [&(MyClassO1.c), &(MyClassO2.c)] by 0x20 (32):

7FD0435D8010	7FD0435D8030

If I change to char[17]

The difference goes up from 0x20 (32) to 0x30 (48), and keeps 
that way until char[32]:

7FD0435D8010	7FD0435D8040

char[33] will increase again by 16 bytes and so on.

Matheus.