Confusion with adj updates in C++ user def function

General
1

I’m trying to write a C++ user defined function which in Stan would be

  vector ode_rhs_c(real time, vector xz,
                 int C_nnz, vector C_w, array[] int C_v, array[] int C_u,
                 real I1, real tau0, real K, vector eta);
    
  vector ode_rhs(real time, vector xz,
                 int C_nnz, vector C_w, array[] int C_v, array[] int C_u,
                 real I1, real tau0, real K, vector eta) {
    int nn = rows(eta);
    vector[nn] x = xz[1:nn];
    vector[nn] z = xz[nn+1:2*nn];
    vector[nn] gx = csr_matrix_times_vector(nn, nn, C_w, C_v, C_u, x);
    vector[nn] dx = 1.0 - x.*x.*x - 2.0*x.*x - z + I1;
    vector[nn] dz = (1/tau0)*(4*(x - eta) - z - K*gx);
    return append_row(dx, dz);
  }

in this gist here,

where the compiler complains about

user_header.hpp:82:14: error: no viable overloaded '+='
    xz.adj() += g_x;
    ~~~~~~~~ ^  ~~~

where g_x is a VectorXd and xz.adj() is a Cwise something or other. I’m a bit confused about how updates to adj() should be correctly implemented as I’ve tried to read the math lib docs and some of the implementations in the math lib, where I thought I was doing the right thing. If anyone could shed some light it would be much appreciated!

edit bonus points for why adj() vs adj_op() vs val_op() … ?

2

Aarrg, it was a const problem: to do x.adj() += gx, x can’t be const or else the compiler complains about seemingly unrelated and irrelevant things.