A unified modeling approach is proposed to cover a broad range of memristive devices. The modular structure of the model enables it to represent behaviors of different types of devices. Resulted from theoretical analyses, the window function is uniquely controlled by the memristive flux. This not only solves the stability problem at boundaries present in previous models, but also reveals that an equivalent charge-flux constitutive relationship can be obtained from various types of memristive devices. Simulations on three device examples show that our model exhibits the device properties such as the frequency-dependent hysteresis, the limited memductance switching range with boundary assurances, the linear/nonlinear dopant drift, and the threshold voltages for read/write mode.