This example illustrates a coding error that is easy to make when creating a SystemVerilog Object-Oriented testbench. Only the relevent portions of code are shown, not the full test context. The test program declares handles for a "Generator" object and a "Scoreboard" object, and then passes those handles to a "build_env" function to actually construct those objects and construct a mailbox so that the generator can communicate with the scoreboard. The problem with this code, however, is that after the "build_env" function returns, the generator and scoreboard handles are still null -- the test program (the "calling scope") does not see the objects that the function created.

program test;
  Generator  gen;  // handle for stimulus generator
  Scoreboard sb;   // handle for response scoreboard
  initial begin
    build_env ( .g(gen), .s(sb) );  // pass handles to environment builder
    ... // gen and sb handles should now reference actual objects, but are null
    ...
  end

  function void build_env(Generator g, Scoreboard s);
    mailbox mbx;
    mbx = new;    // construct a mailbox object and store handle in mbx
    g = new(mbx); // construct a Generator object and store handle in g
    s = new(mbx); // construct a Scoreboard object and store handle in s
    return;
  endfunction
endprogram

Why does it appear that the Generator and Scoreboard objects did not get created?