The builder pattern is the right way to go. It is stronger than const-references, because once the object is created, there is no way to unfreeze it. It guarantees that the object is truly const, no matter what. On the other hand, const keyword in C++ does not guarantee the object is really const. Your reference may be const, but something else might still have a non-const reference and mutate the object.

Most of the time `const` is only needed for the caller to ensure that the object won't get changed when passed to another function.

In addition, to enforce immutability in C++, you have to disable copy/move constructors and assignment operators, which removes much benefit of using values instead of references.

> `const` is only needed for the caller to ensure that the object won't get changed when passed to another function.

But const cannot guarantee this, because it can be cast away so easily. This could be true just for your own code, but then it is you who is responsible for maintaining the immutability, not the compiler.

> because it can be cast away so easily

By this logic, none of the static type checks in C++ is of any use, since they can be cast away easily as well.

Types exist to help you organize your code. Const (for classes) just promotes the misguided idea that you should design objects with mutable and immutable parts at the same time. The main objection to this is that your objects should be either value objects (therefore immutable) or reference objects. There is no need for const if you design your classes in this way. On the other hand, the const idea requires you to spread "const" everywhere you might want immutability, otherwise the compiler will fight you at each line of your code. Either way, it is a loss-loss proposition.

I see this sometimes in code at work. As soon as you need to stuff one of the 'const' objects into any kind of container you start requiring unique_ptrs everywhere, and then you start thinking how much easier your life would be if you wrote Haskell for a living instead.

It that is your intent, then build an immutable class with some friends to initialize the object if necessary. The const thing is so overloaded in C++ that is means close to nothing in terms of intent, and it can be easy circumvented.