Mutable class reference to immutable class

September 10, 2016
Posted by Jonathan Marler
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Jonathan Marler
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This is been bugging me for a while. Is it possible to have a mutable reference to an immutable class?  In other words, can you set a class variable to an immutable class, and then set that variable to another immutable class later?

Mutable "slices" to immutable data are easy:

    immutable(char[]) x = "string for x"; // immutable data x
    immutable(char[]) y = "string for y"; // immutable data y

    immutable(char)[] mutableRef = x; // mutable slice to immutable data
    mutableRef = y; // OK, you can set mutableRef to another immutable slice

But I can't figure out how to make a mutable "class" to immutable classes:

    immutable(SomeClass) x = new immutable SomeClass(); // immutable class x
    immutable(SomeClass) y = new immutable SomeClass(); // immutable class y

    immutable(SomeClass) mutableRef = x;
    mutableRef = y; // Error: cannot modify mutable expression mutableRef


A workaround would be to make mutableRef an immutable(SomeClass)*, but this adds an extra level of indirection.  Since all classes in D are pointers, x is already a pointer, so making a pointer to x would be making a pointer to a pointer that points to a class.

It's obvious this issue is a result of the fact that all class variables are pointers.  I don't suppose there is a way to represent a class's value type that I don't know about is there?

    SomeClass.ValueType?  // Is there semantics for this I don't know about?

If so, you could solve the problem by declaring mutableRef as:

    immmutable(SomeClass.ValueType)* mutableRef = x;

I haven't encountered semantics for this anywhere in the language, but maybe someone else can enlighten me?  If not, is there another way to get a mutable reference to an immutable class?  Thanks in advance for the help.
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