Recently I encounter the following code, it is an excerpt from tutorial of open-source package AMReX text

Array4<Real> const& a = fab1.array();
    Dim3 lo = lbound(a);
    Dim3 hi = ubound(a);
    int nc = a.nComp();
    for(int n=0; n<nc; n++){
        for(int i=lo.x; i<=hi.x; i++){
            for(int j=lo.y; j<=hi.y; j++){
                for(int k=lo.z; k<=hi.z; k++){
                    a(i, j, k, n) = 2.0;
                }
            }
        }
    }

Array4 is a class with template which stores the data of grid points, like matrix, tensor etc. I am wondering why here variable “a” as a reference referring to a constant object can be used to modify the object’s data. What are the reasons behind it? Thanks!

The tutorial says that “const means the Array4 object itself cannot be modified to point to other data”. So why Array4 object can be modified here?