Which one is preferred, return const double& OR return double

Question

Given the following scenario, which one of the following is preferred. m_state is a member rater than a local variable.

class C
{
private:
    double m_state;
public:
    double state() const { return m_state; } // returns double
    double& state() { return m_state; }
}

===========================================

class C
{
private:
    double m_state;
public:
    const double& state() const { return m_state; } // returns const double&
    double& state() { return m_state; }
}

Answer 1

I wouldn't do this:

double& state() { return m_state; }

You may as well make m_state public if you did that. Probably what makes the most sense is:

const double & state() const { return m_state; }

Then again, when you're talking about saving the copy of a 64 bit variable (ie micro-optimization) and the fact that the latter version can be recast to whatever you want, I would just copy it:

double state() const { return m_state; }

(not that there's any true security anyway)