Expose_stan_functions C++ code not compliling

Modeling
6

I now have code that works:

functions {

 
// Example integrand
  real[] integrand_ode(real y, real[] f, real[] theta, real[] x, int[] x_i) {
    real df_dx[1]; // Do not change
    df_dx[1] = exp(-y^2 - x[1]^2); // Function of x and y to be integrated
    return(df_dx);
  }
//User-defined function of x after y integrated out
  real[] first_integral(real x, real[] f, real[] theta, real[] x_r, int[] x_i) {     
    return(integrate_ode_rk45(integrand_ode, 
                            rep_array(0.0, 1), // Do not change
                            0.0, // ymin
                            rep_array(1.0, 1), //(ymax, 1)
                            rep_array(0.0, 0), // Do not change
                            rep_array(x, 1), // Do not change
                            x_i)[1,]);// Do not change
  }
  // Final integration wrt x
  real second_integral(real xmin, real xmax ) {
    int x_i[0];
    return(integrate_ode_rk45(  first_integral, 
                            rep_array(0.0, 1),// Do not change
                            xmin,   //xmin
                            rep_array(xmax, 1), // (xmax, 1)
                            rep_array(0.0, 0),// Do not change
                            rep_array(0.0, 0) , // Do not change
                            x_i)[1,1]);// Do not change
  }
}
data {
}
model {}