CODEWITHSUNDEEP
f#
colinfang
colinfang

Reputation: 21767

Is it possible for an object to access private field / function of another object of same class?

I know this is possible in C#, which produces simple and efficient code. --- Two objects of the same class can access each other's private parts.

class c1
{
    private int A;

    public void test(c1 c)
    {
        c.A = 5;
    }
}

But It seems impossible in F#, is it true?

type c1()
     let A = 0
     member test (c: c1) = c.A

Upvotes: 7

Views: 752

Answers (5)

John Palmer
John Palmer

Reputation: 25526

You just use a directly in an instance method 

type c1()
     let A = 0
     member x.test = A

For a static method this doesn't work as let bindings are slightly different - then you need a class definition like

type c1()
     private member x.A = 0
     static member test (A:c1) = A.A

Upvotes: 1

Be Brave Be Like Ukraine
Be Brave Be Like Ukraine

Reputation: 7735

This is possible and it is widely used, for example, for checking memberwise equality:

type c1 =
    member private this.A = 0
    interface IEquatable<c1> with
        member this.Equals (that: c1) = this.A = that.A
    // of course, it can be done in a regular method as well
    member this.Equals (that: c1) = this.A = that.A

Upvotes: 1

kvb
kvb

Reputation: 55185

As section 8.6.1.3 of the F# spec states:

The functions and values defined by instance definitions are lexically scoped (and thus implicitly private) to the object being defined.

Upvotes: 1

afrischke
afrischke

Reputation: 3866

Interesting question. It seems to work with an explicit field but not with a let binding:

// Works
type c1 =
    val private A : int
    new(a) = { A = a }
    member m.test(c : c1) = c.A

let someC1 = new c1(1)
let someMoreC1 = new c1(42);
let theAnswer = someC1.test someMoreC1

// Doesn't work
type c2() =
    let mutable A = 42
    // Compiler error: The field, constructor or member 'A' is not defined
    member m.test(c : c2) = c.A

Upvotes: 6

Stephen Swensen
Stephen Swensen

Reputation: 22307

Yes, but in your example A is not semantically a private member of c1, it is more like a local variable of the constructor.

@afrischke gives an example of how to define c1 with an actual private member A (using val fields).

Upvotes: 2

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