Where does the get_input_port method reside in drake?

I'm trying to find the template function that this call is referring to

https://github.com/RobotLocomotion/drake/blob/master/examples/pendulum/pendulum_geometry.cc#L102

  const auto& input = get_input_port(0).Eval<PendulumState<double>>(context);

I've been trying to find the import that contains this call but I'm at a loss.