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
>     }
> }

On Thursday, 15 March 2018 at 10:16:49 UTC, 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
>>     }
>> }

class A {
    private int _value = 12;

    int value() @property { return _value; }
    void updateValue() { value = 13; }
}

...
auto a = new A();
writeln(a.value);
a.updateValue();
writeln(a.value);

On Thursday, 15 March 2018 at 10:16:49 UTC, 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
>>     }
>> }

You can do this:

class B {
    private A _a;
    @property const(A) a() { return _a; }

    this() {
        _a = new A();
        _a.updateValue();
    }
}

--
  Simen

On Thursday, 15 March 2018 at 10:55:16 UTC, Mike Parker wrote:

>
> class A {
>     private int _value = 12;
>
>     int value() @property { return _value; }
>     void updateValue() { value = 13; }
> }
>
> ...
> auto a = new A();
> writeln(a.value);
> a.updateValue();
> writeln(a.value);

Sorry. I overlooked that B.a is const.

On Thursday, 15 March 2018 at 10:57:52 UTC, Mike Parker wrote:
> On Thursday, 15 March 2018 at 10:55:16 UTC, Mike Parker wrote:
>
>>
>> class A {
>>     private int _value = 12;
>>
>>     int value() @property { return _value; }
>>     void updateValue() { value = 13; }
>> }
>>
>> ...
>> auto a = new A();
>> writeln(a.value);
>> a.updateValue();
>> writeln(a.value);
>
> Sorry. I overlooked that B.a is const.

It still works, the `value` just needs to be `const` (or `inout`) and _value needs to be used in updateValue:

class A {
    private int _value = 12;

    int value() const { return _value; }
    void updateValue() { _value = 13; }
}


auto a = new A();
writeln(a.value);
a.updateValue();
writeln(a.value);


In action: https://run.dlang.io/is/Tk1rY1

@Robert: If you need this a lot, the accessors package might be interesting to you:

https://code.dlang.org/packages/accessors

On Thursday, 15 March 2018 at 13:44:20 UTC, Seb wrote:
> On Thursday, 15 March 2018 at 10:57:52 UTC, Mike Parker wrote:
>> On Thursday, 15 March 2018 at 10:55:16 UTC, Mike Parker wrote:
>>
>>> [...]
>>
>> Sorry. I overlooked that B.a is const.
>
> It still works, the `value` just needs to be `const` (or `inout`) and _value needs to be used in updateValue:
>
> class A {
>     private int _value = 12;
>
>     int value() const { return _value; }
>     void updateValue() { _value = 13; }
> }
>
>
> auto a = new A();
> writeln(a.value);
> a.updateValue();
> writeln(a.value);
>
>
> In action: https://run.dlang.io/is/Tk1rY1
>
> @Robert: If you need this a lot, the accessors package might be interesting to you:
>
> https://code.dlang.org/packages/accessors

(I meant @Andrey - sorry)

On Thursday, 15 March 2018 at 13:44:20 UTC, Seb wrote:
> On Thursday, 15 March 2018 at 10:57:52 UTC, Mike Parker wrote:
>> On Thursday, 15 March 2018 at 10:55:16 UTC, Mike Parker wrote:
>>
>>>
>>> class A {
>>>     private int _value = 12;
>>>
>>>     int value() @property { return _value; }
>>>     void updateValue() { value = 13; }
>>> }
>>>
>>> ...
>>> auto a = new A();
>>> writeln(a.value);
>>> a.updateValue();
>>> writeln(a.value);
>>
>> Sorry. I overlooked that B.a is const.
>
> It still works, the `value` just needs to be `const` (or `inout`) and _value needs to be used in updateValue:
>
> class A {
>     private int _value = 12;
>
>     int value() const { return _value; }
>     void updateValue() { _value = 13; }
> }
>
>
> auto a = new A();
> writeln(a.value);
> a.updateValue();
> writeln(a.value);
>
>
> In action: https://run.dlang.io/is/Tk1rY1
>
> @Robert: If you need this a lot, the accessors package might be interesting to you:
>
> https://code.dlang.org/packages/accessors

... and I didn't add `const` to A

-> https://run.dlang.io/is/QObN1w

but better have a look at Simen's response (https://forum.dlang.org/post/ottyywbmrwgfabgpfehk@forum.dlang.org), it's what you are looking for. I read too quickly and only have of it :/

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