How to write C++ getters and setters

Question

If I need to write a setter and/or getter for a property I write it like this:

struct X { /*...*/};

class Foo
{
private:
    X x_;

public:
    void set_x(X value)
    {
        x_ = value;
    }
    X get_x()
    {
        return x_;
    }
};

However I have heard that this is the Java style of writing setters and getters and that I should write it in C++ style. Moreover I was told it is ineficient and even incorrect. What does that mean? How can I write the setters and getters in C++?

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Assume the need for getters and/or setters is justified. E.g. maybe we do some checks in the setter, or maybe we write only the getter.

Answer 1

There are two distinct forms of "properties" that turn up in the standard library, which I will categorise as "Identity oriented" and "Value oriented". Which you choose depends on how the system should interact with Foo. Neither is "more correct".

Identity oriented

class Foo
{
     X x_;
public:
          X & x()       { return x_; }
    const X & x() const { return x_; }
}

Here we return a reference to the underlying X member, which allows both sides of the call site to observe changes initiated by the other. The X member is visible to the outside world, presumably because it's identity is important. It may at first glance look like there is only the "get" side of a property, but this is not the case if X is assignable.

 Foo f;
 f.x() = X { ... };

Value oriented

class Foo
{
     X x_;
public:
     X x() const { return x_; }
     void x(X x) { x_ = std::move(x); }
}

Here we return a copy of the X member, and accept a copy to overwrite with. Later changes on either side do not propagate. Presumably we only care about the value of x in this case.