C++ 在继承的 class 中访问运算符 function

Question

I am currently writing an application where I use the Eigen-Library for complex 3D-Math. Because I needed distinct point and vector classes, my point3d-class looks like this:

class point3d : public Eigen::Vector4d
{
public:
    point3d(){}
    point3d(double x, double y, double z) : Eigen::Vector4d(x, y, z, 1) {}
    void fromString(std::string input);
};

Now I want to create a member function of this class that allows me to parse lines of OBJ-files which look like this:

v 2.8 0.52 10.18

as such point. This is how I intend to design my parsing function

void point3d::fromString(std::string input)
{
    char* buf = strdup(input.c_str());
    if (strtok(buf, " ") == "v")
    {

        ;
        strtok(buf, " ");
        this-x = std::stod(strtok(buf, " "));
        this->y = std::stod(strtok(buf, " "));
        this->z = std::stod(strtok(buf, " "));  

    }
}

My problem is that Eigen does not allow me to access the data stored in Vector4d as this->x, this-y, this->z and so on. Instead, you'd usually access it as Vector4d v; v[0], v[1], v[2] etc. I think the way it does that is by using a

double Eigen::Vector4d::operator[](unsigned int index){...}

function.

I don't know how exactly one would access that data in a derived class. What do I have to do to access this data within that function so that I can write to the x, y, z values?

Answer 1

You can do

(*this)[0]

and so on in order to call the base class operator[] .

Answer 2

x , y , z aren't member variables of Eigen::Vector4d , but methods. You can access them from your derived class using this->x() etc (or Vector4d::x() ). What also works is (*this)[0] or (*this)(0) . If you need to access x() , y() , z() frequently in your class, you could write once:

using Vector4d::x;
using Vector4d::y;
using Vector4d::z;

And afterwards access them using just x() , y() , z() . However, this may cause some shadowing conflicts (if you name local variables the same way).