• Adnan (16/16) Feb 18 2020 I have a base class that has a couple of constant member
• Simen =?UTF-8?B?S2rDpnLDpXM=?= (20/36) Feb 18 2020 const members can only be set in the constructor of the type that
• Petar Kirov [ZombineDev] (11/27) Feb 18 2020 `const` members must be initialized by the same class that

Adnan <relay.public.adnan outlook.com> writes:

I have a base class that has a couple of constant member 
variables. These variables are abstract, they will only get 
defined when the derived class gets constructed.

class Person {
     const string name;
     const int id;
}

class Male : Person {
     this(string name = "Unnamed Male") {
         static int nextID = 0;
         this.id = nextID++;
         this.name = name;
     }
}

The compiler restricts me from assigning those two functions. How 
can I get around this?

Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:

On Tuesday, 18 February 2020 at 12:37:45 UTC, Adnan wrote:
 I have a base class that has a couple of constant member 
 variables. These variables are abstract, they will only get 
 defined when the derived class gets constructed.

 class Person {
     const string name;
     const int id;
 }

 class Male : Person {
     this(string name = "Unnamed Male") {
         static int nextID = 0;
         this.id = nextID++;
         this.name = name;
     }
 }

 The compiler restricts me from assigning those two functions. 
 How can I get around this?

const members can only be set in the constructor of the type that 
defines them. To set them in a subclass, forward the values to 
the superclass' constructor:

class Person {
     const string name;
     const int id;
     protected this(string _name, int _id) {
         id = _id;
         name = _name;
     }
}

class Male : Person {
     this(string name = "Unnamed Male") {
         static int nextID = 0;
         super(name, nextID++);
     }
}

--
   Simen

Petar Kirov [ZombineDev] <petar.p.kirov gmail.com> writes:

On Tuesday, 18 February 2020 at 12:37:45 UTC, Adnan wrote:
 I have a base class that has a couple of constant member 
 variables. These variables are abstract, they will only get 
 defined when the derived class gets constructed.

 class Person {
     const string name;
     const int id;
 }

 class Male : Person {
     this(string name = "Unnamed Male") {
         static int nextID = 0;
         this.id = nextID++;
         this.name = name;
     }
 }

 The compiler restricts me from assigning those two functions. 
 How can I get around this?

`const` members must be initialized by the same class that 
declares them. What you could do is have the abstract Person 
class declare a constructor (which would initialize the `const` 
members) and call it from derived class (such as `Male`) 
constructors by the `super(arg1, arg2)` syntax.

Alternatively, you could define `abstract` accessor functions in 
the base class and have the derived classes implement them. In D 
you can use the same syntax to call functions as if they were 
fields. (Before you had to put the  property attribute on such 
functions, but for the most part it is not necessary now.)