Question

I have a ShapeType, Point, with some coordinates, (1,2), and I’d like to use apply_visitor in an overloaded operator () to add the coordinates (3,4) to my Point, so that Point ends up being (4,6). Where is my implementation failing? I think my ShapeVisitor class is correct, but I am getting an error, “apply_visitor” is not a member of CLARK::Point.

Code is as follows.

#include "Point_H.hpp"
#include "Shape_H.hpp"
#include "boost/variant.hpp"

typedef boost::variant<Point,Line,Circle> ShapeType;

ShapeType ShapeVariant(){...}

class ShapeVisitor : public boost::static_visitor<> 
{
private:
    double m_dx; // point x coord
    double m_dy; // point y coord

public:    
    ShapeVisitor(double m_dx, double m_dy);
    ~ShapeVisitor();

    // visit a point
    void operator () (Point& p) const
    {
        p.X(p.X() + m_dx);
        p.Y(p.Y() + m_dy);
    }
};

int main()
{   
    using boost::variant;

    ShapeType myShape = ShapeVariant(); // select a Point shape

    Point myPoint(1,2);

    boost::get<Point>(myShape) = myPoint; // assign the point to myShape

    boost::apply_visitor(ShapeVisitor(3,4), myPoint); // trying to add (3,4) to myShape

    cout << myPoint << endl;

    return 0;
}

Thanks!

Answer 1

1. You are missing the include (Edit: doesn’t seem to be required anymore)

   #include "boost/variant/static_visitor.hpp"
   

2. Also instead of

   boost::get<Point>(myShape) = myPoint;
   

   You’ll just want to do

   myShape = myPoint;
   

   Otherwise, if the variant did not actually contain a Point yet, you’ll receive a boost::bad_get exception

3. Finally

   boost::apply_visitor(ShapeVisitor(3,4), myPoint);
   

   should have been

   boost::apply_visitor(ShapeVisitor(3,4), myShape);
   

A simple self-contained example that shows all these points would look like this:
(See it live on http://liveworkspace.org/code/33322decb5e6aa2448ad0359c3905e9d)

#include "boost/variant.hpp"
#include "boost/variant/static_visitor.hpp"

struct Point { int X,Y; };

typedef boost::variant<int,Point> ShapeType;

class ShapeVisitor : public boost::static_visitor<> 
{
private:
    double m_dx; // point x coord
    double m_dy; // point y coord

public:    
    ShapeVisitor(double m_dx, double m_dy) : m_dx(m_dx), m_dy(m_dy) { }

    void operator () (int& p) const { }

    // visit a point
    void operator () (Point& p) const
    {
        p.X += m_dx;
        p.Y += m_dy;
    }
};

int main()
{   
    Point myPoint{ 1,2 };

    ShapeType myShape(myPoint);
    boost::apply_visitor(ShapeVisitor(3,4), myShape);

    myPoint = boost::get<Point>(myShape);
    std::cout << myPoint.X << ", " << myPoint.Y << std::endl;
}

Output:

4, 6