• Nrgyzer (18/18) Aug 07 2017 Hi guys,
• Mike Parker (14/32) Aug 07 2017 Don't use pointers. Classes are already reference types:

Nrgyzer <nrgyzer gmail.com> writes:

Hi guys,

I've the following code:

abstract class a {}
class b : a { this(a* myAttr = null) {} }
class c : a { this(a* myAttr = null) {} }

void main()
{
     auto myb = new b();
     auto myc = new c(&myb);
}

DMD says "Constructor c.this(a* myAttr = null) is not callable 
using argument types (b*)". I'm confused why it fails because 
class b and c are both derived from class a and the constructor 
of class b accepts a pointer of class a.

When I replace "auto myb = new b();" by "a myb = new b();", it 
works as expected. But then I cannot class-specific functions of 
class b because I've the instance of the base-class a. So, what's 
the correct way?

Mike Parker <aldacron gmail.com> writes:

On Tuesday, 8 August 2017 at 06:03:06 UTC, Nrgyzer wrote:
 Hi guys,

 I've the following code:

 abstract class a {}
 class b : a { this(a* myAttr = null) {} }
 class c : a { this(a* myAttr = null) {} }

 void main()
 {
     auto myb = new b();
     auto myc = new c(&myb);
 }

 DMD says "Constructor c.this(a* myAttr = null) is not callable 
 using argument types (b*)". I'm confused why it fails because 
 class b and c are both derived from class a and the constructor 
 of class b accepts a pointer of class a.

 When I replace "auto myb = new b();" by "a myb = new b();", it 
 works as expected. But then I cannot class-specific functions 
 of class b because I've the instance of the base-class a. So, 
 what's the correct way?

Don't use pointers. Classes are already reference types:

abstract class a {}
class b : a { this(a myAttr = null) {} }
class c : a { this(a myAttr = null) {} }

void main()
{
     auto myb = new b();
     auto myc = new c(myb);
}

If you pass class pointers around, you're passing pointers to the 
*references*, not to the *instances*. A b is always an a, but a 
b* is not an a* unless you cast. And anyway, it's almost 
certainly not what you want.