Passing temporary objects by reference

Question

I have two classes: Circle and Dot

Class Circle contains a Dot, and Dot contains an int. Circle has a getDot() function, and Dot has a lessThan(Dot& ) function.

I want to sort an array of Circles, elems[], by the int values inside Dot. If I have a Circle circ that I want to compare to some value in the array, I essentially need a line that does the following:

elems[0].getDot().lessThan(circ.getDot())

But it won't compile... I can fix the issue by having a temporary

Dot dt = circ.getDot()

and passing

elems[0].getDot().lessThan(dt)

but this seems like unnecessary copying. Is there a more efficient way to solve this?

Unfortunately, I am limited to using lessThan(dot&) for comparison, but I can modify the contents of it.

The compiler error is: error: invalid initialization of non-const reference of type ‘Dot&’ from an rvalue of type ‘Dot’ cout<

Answer 1

example:

#include <vector>
#include <algorithm>

struct Dot
{
    Dot(int i) : value_(i) {}
    bool lessThan(Dot const& other) const
    {
        return value_ < other.value_;
    }

    int value_;
};

struct Circle
{
    Circle(Dot dot = Dot(0))
    : dot_(dot)
    {
    }

    const Dot& getDot() const { return dot_; }

    Dot dot_;
};


void sortCircles(std::vector<Circle>& circles)
{
    auto order = [](Circle const& l, Circle const& r)
    {
        return l.getDot().lessThan(r.getDot());
    };
    std::sort(circles.begin(), circles.end(), order);
}