Can Scala call by reference?

scala calling-convention evaluation-strategy
royco picture royco · Jan 25, 2011 · Viewed 28.9k times · Source

I know that Scala supports call-by-name from ALGOL, and I think I understand what that means, but can Scala do call-by-reference like C#, VB.NET, and C++ can? I know that Java cannot do call-by-reference, but I'm unsure if this limitation is solely due to the language or also the JVM.

This would be useful when you want to pass an enormous data structure to a method, but you don't want to make a copy of it. Call-by-reference seems perfect in this case.

Answer

Rex Kerr picture Rex Kerr · Jan 25, 2011

Java and Scala both use call by value exclusively, except that the value is either a primitive or a pointer to an object. If your object contains mutable fields, then there is very little substantive difference between this and call by reference.

Since you are always passing pointers to objects not the objects themselves, you don't have the problem of having to repeatedly copy a giant object.

Incidentally, Scala's call by name is implemented using call by value, with the value being a (pointer to a) function object that returns the result of the expression.

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