Check out this header-only C++11 library of control and filter tools. Currently it features PID control, bi-quads (or second-order-sections), state-space systems and a PRBS (pseudo-random-binary-signal) system identification function.
The code-documentation and tests provide reference and example to use the library. Creating a second order floating-point state-space is easy:
using ss = control::system::ss<float,2>; // State-transfer matrix ss::TA A; A << 1, 1, 0, 1; // Input matrix ss::TB B; B << 0.5, 1; // Output matrix ss::TC C; C << 1, 0; // Feedtrough matrix ss::TD D; D << 0; ss *P = new ss(A,B,C,D);
Under the hood, it uses the Eigen matrix library to allow vectorisation and optimisations.
Find it on Github at tomlankhorst/control.