What is the C++ equivalent of C#'s readonly field modifier? [duplicate]

Question

Locking down state is great. In C# you can ensure that a field doesn't change it's value/reference once the constructor completes by declaring it as readonly.

class Foo
{
    private readonly string _foo;

    public Foo() {
        _foo = "Unchangeable";
    }

    public void ChangeIt() {
        _foo = "Darn";        // compiler error
    }
}

Can I do the same thing with C++? If so, how? If not, why not?

Answer 1

That would be const. Note that this keyword means a couple of different things in different contexts.

Answer 2

class Foo
{
private:
    const string _foo;
public:
    Foo() : _foo("Unchangeable")
    {
    }
    void ChangeIt()
    {
        _foo = "Darn";        // compiler error
    }
};

Answer 3

There is no such thing directly. You can use a private field with a public getter (but no setter). But that would only apply to other classes calling your code. Foo always has full acces to its members. But since you are the implementer of Foo, this is no real problem.