What is the meaning of Eigen's .resize(rows,cols) Matrix const MatrixXd as 'this' argument discards qualifiers error

Question

When the Matrix argument is defined as constant I get

.resize(rows,cols) Matrix const MatrixXd as 'this' argument discards qualifiers error

    void (const Eigen::MatrixXd &X){
    X.resize(cols, rows) }

returns an error but this works as expected:

    void(Eigen::MatrixXd &X){
    X.resize(cols, rows)}

I'm not too familiar with c++ (other than using it for this class) and am wondering what this means?

Thanks for any pointers.

Answer 1

The warning means that the resize function that you called is not const qualified. The lack of const qualification means that the function cannot be called on a const lvalue. resize is a function that modifies the object. The rough meaning of "const" is that modification isn't allowed.

X is an lvalue reference to const, so non-const qualified functions cannot be called through the reference. You attempted to call a non-const qualified function through the const reference. Since that's not allowed, the compiler told you about the bug.