Immutable "functional" data structure in C++11

Question

I was trying to write down some implementations for a couple of data structures that I'm interested in for a multithreaded / concurrent scenario.

A lot of functional languages, pretty much all that I know of, design their own data structures in such a way that they are immutable, so this means that if you are going to add value to an instance t1 of T, you really get a new instance of T that packs t1 + value.

 container t;
 container s = t; //t and s refer to the same container.
 t.add(value); //this makes a copy of t, and t is the copy

I can't find the appropriate keywords to do this in C++11; there are keywords, semantics and functions from the standard library that are clearly oriented to the functional approach, in particular I found that:

• mutable it's not for runtime, it's more likely to be an hint for the compiler, but this keyword doesn't really help you in designing a new data structure or use a data structure in an immutable way
• swap doesn't works on temporaries, and this is a big downside in my case

I also don't know how much the other keywords / functions can help with such design, swap was one of them really close to something good, so I could at least start to write something, but apparently it's limited to lvalues .

So I'm asking: it's possible to design immutable data structure in C++11 with a functional approach ?

Answer 1

You simply declare a class with private member variables and you don't provide any methods to change the value of these private members. That's it. You initialize the members only from the constructors of the class. Noone will be able to change the data of the class this way. The tool of C++ to create immutable objects is the private visibility of the members.

mutable: This is one of the biggest hacks in C++. I've seen at most 2 places in my whole life where its usage was reasonable and this keyword is pretty much the opposite of what you are searching for. If you would search for a keyword in C++ that helps you at compile time to mark data members then you are searching for the const keyword. If you mark a class member as const then you can initialize it only from the INITIALIZER LIST of constructors and you can no longer modify them throughout the lifetime of the instance. And this is not C++11, it is pure C++. There are no magic language features to provide immutability, you can do that only by programming smartly.