如何在C ++中自己的类中正确使用矩阵（本征库）？

Question

I really tried to solve this problem and I have read a lot of QAs here on Stackoverflow but somehow nothing really helped.

I am trying to implement a Class with its own matrices and vectors from the Eigen library. I am using Code::Blocks with GNU GCC Compiler.

Here is a simple example of what I mean, but it is not the exact code, because I am using more matrices and vectors and of other sizes (4x4 Matrices, 2x4 Matrices, 4x1 Vectors and 2x1 Vectors):

    class MYCLASS{
        private:

            VectorXd x;
            MatrixXd A;

        public:

            MYCLASS(double, double, double, double);
            double get_matval();

    };

    MYCLASS::MYCLASS(double deltaT, double q_var, double r1_var, double r2_var){

        x_m(0)=q_var;
        x_m(1)=r1_var;
        x_m(2)=r2_var;
        x_m(3)=0.0;

        A(0,0)= deltaT;
        A(0,1)= 0.0;
        A(1,0)= 0.0;
        A(1,1)= 0.0;

    }

    double MYCLASS::get_matval(){

        return A(0,0);

    }

1. When I create an object of MYCLASS, like this:

MYCLASS myobject(10, 0.5, 0.1, 0.75);

==> Compilation is good, but when it runs, then the program somehow crashes with the following exact description (keep in mind I have other matrices and vectors):

Assertion failed: index >= 0 && index < size(), file F:....../Eigen/src/Core/DenseCoeffsBase.h, line 425

This application has requested the Runtime to terminate it in an unusual way. Please contact...

Process returned 3 (0x3) execution time: 2.131 s

2. If I then put at the beginning of the constructor this:

    MYCLASS::MYCLASS(double deltaT, double q_var, double r1_var, double r2_var){

            VectorXd x(4);
            MatrixXd A(2,2);

            x_m(0)=q_var;
            x_m(1)=r1_var;

            ...and so on...

==> then when constructing the object ==> no error

==> but when I then want to access A(0,0) by using the function get_matval, like this:

MYCLASS myobject(10, 0.5, 0.1, 0.75);

double myvar = myobject.get_matval();

...it crashes again with the same error message.

Could you guys please help?

Answer 1

Even though you found a solution already, here is how to properly initialize (a limited number of) Eigen matrices:

MYCLASS::MYCLASS(double deltaT, double q_var, double r1_var, double r2_var)
                : x(4), A(2,2) { // use initializer list to construct x and A
    x << q_var, r1_var, r2_var, 0.0;
    A << deltaT, 0.0, 0.0, 0.0;
}