try.cpp

template<typename DecomposableFunctionType, typename DecayType = SomeValue>
class SGD
{
...
...
...
...
private:
...
}
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Policy Class
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/**
 * Implementation of the NoDecaySGD Policy
 */
class NoDecay
{
  public:

  NoDecay(){}

  /**
   * Decay the learning rate based on the policy
   * 
   * @param stepSize: Step size of SGD
   * @param decay_rate: Decay rate for the step size
   * @param decay_step_size: Step size for the step policy
   * @param iteration: Iteration index
   */
  inline double decay_learning_rate(double stepSize,
                                    double decay_rate, 
                                    int decay_step_size,
                                    int iteration);
  {
    return stepSize;
  }
};//similarly i have implement 3 other policies
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sgd_impl.hpp
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template<typename DecomposableFunctionType, typename DecayType = SomeValue>
SGD<DecomposableFunctionType>::SGD(DecomposableFunctionType& function,
...
...
{
  using DecayType::decay_learning_rate;
...
...
stepSize = decay_learning_rate(stepSize, decay_rate, decay_step_size, i)
iterate -= stepSize  * gradient;
...
...
}