CODEWITHSUNDEEP
scala
user48956
user48956

Reputation: 15788

Member-wise assignment in scala?

Does scala support memberwise assignment?

Given:

case class C(var x:Int, var y:Int)
val c = C(1,2)
val d = C(3,4)

is there an operator to assign each member of d to c.

In C/C++ you would have:

struct C{ int x, int y)
C c = {1,2}
C d = {3,4}

c = d

edit1

One of the great benefits of member-wise assignment is that its automatic, in

Both

c() = d

and

c.copyFrom( d )

both suffer from a maintenance problem - if new members are added members to C its easy to overlook adding the member to the user-created function. What's really desired is automated way to copy these values.

edit2

Another use case for this behavior:

val props:ThirdPartyType = ThirdPartyLibrary.getProperties()
val myprops:ThirdPartyType = MyLibrary.loadPropertiesFromFile()
props @= myprops  // magic member-wise assignment

Here we may find:

I can do:

props.a = myprops.a
props.b = myprops.b
...

but this pattern break when we update to V2 of the ThirdParty library. ThirdPartyType has gained new members that we didn't not copy.

Can't this be solved through reflection?

Upvotes: 1

Views: 123

Answers (2)

som-snytt
som-snytt

Reputation: 39577

The enhancement is easy:

scala> :pa
// Entering paste mode (ctrl-D to finish)

case class C(var x:Int, var y:Int)
val c = C(1,2)
val d = C(3,4)

// Exiting paste mode, now interpreting.

defined class C
c: C = C(1,2)
d: C = C(3,4)

scala> implicit class CUpdater(val c: C) { def update(d: C) = { c.x=d.x ; c.y=d.y } }
defined class CUpdater

scala> c() = d

scala> c
res1: C = C(3,4)

Usual caveats around mutability apply. And you didn't hear it from me.

Upvotes: 5

Robin Green
Robin Green

Reputation: 33063

This will get you a separate object with all case class constructor arguments copied over, although it will not change the original object that d pointed to:

d = c.copy()

If you really want to modify the original object that d points to (perhaps because you have a reference to it somewhere else - think of pointers to structs in the C programming language), then I think you would have to do something fancy with metaprogramming (i.e. reflection or macros) to get a general solution for this.

But I recommend programming with immutable vals, almost always! If you can live with that, copy is fine.

Upvotes: 3

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