Re: Readonly field for class type

[Ali Çehreli via Digitalmars-d-learn]

On 03/15/2018 03:16 AM, Andrey wrote:

Hello, is there way to declare read only field for class type with ability to call inner non constant methods? i.e.:

class A {
    int value = 12;
    void updateValue() {
        value = 13;
    }
}

class B {
    const A a;

    this() {
        a = new A();

        a.updateValue();  // error: mutable method is not callable using const object

    }
}

Another option is assigning a pre-made A to the member:

class A {
    int value = 12;
    void updateValue() {
        value = 13;
    }
}

// Magic recipe is moved here:
A makeA() {
    auto a = new A();
    a.updateValue();
    return a;
}

class B {
    const A a;

    this() {
        a = makeA();
    }
}

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

Which can be compressed into a lambda in the constructor:

    this() {
        a = {
            // This 'a' is a local variable inside the lambda:
            auto a = new A();
            a.updateValue();
            return a;
        }();
    }

Ali