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digitalmars.D.learn - Constructor is not callable using argument types

reply 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?
Aug 07
parent 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.
Aug 07