c++ stateful functor that populates vectors

Question

I have a question regarding functors. I built a simple class:

    class PolygonPrinter { 
        private:
            std::vector<float> x;
            std::vector<float> y;
        public:
            inline void operator()(Point& p) {
                x.push_back(boost::geometry::get<0>(p));
                y.push_back(boost::geometry::get<1>(p));
            }

            void printPoints() {
                for(int i=0; i < x.size(); i++) {
                    std::cout << "(" 
                              << x[i] << "," << y[i] 
                              << ")" << std::endl;
                }
            }
    }

Which I wanted to use as a functor. This is used in something like

    PolygonPrinter<point_2d> polyPrinter;
    boost::geometry::for_each_point( polygon, polyPrinter );
    polyPrinter.printPoints();

Now it seems the functor part works fine as I see the vectors being populated with all the polygon elements (so the for_each_point works as expected), however, the third call ( printPoints ) prints no points and in fact both vectors are empty. I guess this behaviour is expected, however, I cannot understand how the vectors are cleared. I thought you could have stateful functors.

Why are the vector fields x and y in the polyPrinter instance empty?

Answer 1

std algorithms copy your functor. boost probably does the same.

You can std::ref( functor ) to pass it via pseudo-ref, and you'll get the behaviour you want.

boost::geometry::for_each_point( polygon, std::ref(polyPrinter) );

As an aside, your use of inline is redundant: all methods defined in the body of a class are implicitly inline. This is the source of some fun to track down bugs.