Re: Construct immutable member in derived class

[Simen Kjærås via Digitalmars-d-learn]

On Wednesday, 4 April 2018 at 10:11:37 UTC, Timoses wrote:
Example:
```
class A
{
    immutable int i;

    this(){}
}

class B : A
{
    this()
    {
        this.i = 3;
    }
}

void main()
{
    auto b = new B;
}
```

throws:
Error: constructor `onlineapp.A.this` missing initializer for 
immutable field i
Error: cannot modify immutable expression this.i

Why can't I initialize the immutable member in the derived 
class?

Because by the time B's constructor is called, A might already 
have initialized it, and rely on it never changing. The solution 
is to add a constructor overload to A, and call that from B:

class A
{
    immutable int i;

    protected this(int i) {
        this.i = i;
    }
}

class B : A
{
    this()
    {
        super(3);
    }
}

unittest
{
    auto b = new B;
}

--
  Simen