digitalmars.D.learn - Mutable class reference to immutable class
Jonathan Marler <johnnymarler gmail.com> writes:
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.
Sep 10 2016
Lodovico Giaretta <lodovico giaretart.net> writes:
On Saturday, 10 September 2016 at 19:46:51 UTC, Jonathan Marler wrote:This is been bugging me for a while. Is it possible to have a mutable reference to an immutable class? [...]You are probably looking for std.typecons.Rebindable: https://dlang.org/phobos/std_typecons.html#.Rebindable
Sep 10 2016