Re: [Boost-users] Troubles using boost::circular_buffer

Subject: Re: [Boost-users] Troubles using boost::circular_buffer
From: Daryle Walker (darylew_at_[hidden])
Date: 2012-04-17 05:09:35

From: r.ted.byers_at_[hidden]
Date: Thu, 12 Apr 2012 23:54:41 -0400

 
In the header that provides the prototypes for the functions in this particular CPP file, I have:
 
double calculateStdDev(const boost::circular_buffer<double>& input, double& mean, double& variance, double& stddev);
 
And gcc v 4.3.4 gives me the following error:
 
../../include/distribution.moments.hpp:9: error: wrong number of template arguments (1, should be 2)
/usr/local/include/boost/circular_buffer/base.hpp:70: error: provided for 'template<class T, class Alloc> class boost::circular_buffer'
 
I am not sure I understand why. 
The circular_buffer class template does take two arguments, but the forwarding header supplies a default argument to the second one (std::allocator<T>).  Somehow that forward declaration is being missed.

Does this mean that I have to make calculateStdDev a template function?  E.g.: template <typename T>double calculateStdDev(const boost::circular_buffer<double, T>& input, double& mean, double& variance, double& stddev) {                // and put the entire implementation here in the header instead of in the cpp file with the overloads for other types such as std::vector)} No, your code computation code isn't the problem.
But, if that is the case, why is it possible to instantiate such a buffer using the following (as in the example)? boost::circular_buffer<int> cb(3);  I must have missed something, but I don't know what.
Yeah, that part of your code is seeing the defaulted argument.
I looked up standard-deviation in Wikipedia, and I think you should compute your data in a different way:
// Not testedclass mean_and_stddev_computer{    double  m_, q_;    unsigned long  c_;
public:    typedef double  argument_type, result_type;
    mean_and_stddev_computer()        : m_(), q_(), c_()    {}    void  operator ()( double sample )    {        double const  delta = sample - m_;        m_ += delta / ++c_;        q_ += delta * ( sample - m_ );    }
   
double  sample_variance() const  { return (c_ > 1u) ? (q_ / (c_ - 1)) : 0; }    double  standard_variance() const  { return c ? (q_ / c_) : 0; }   
double  sample_stddev() const  { return std::sqrt(sample_variance()); }   
double  standard_stddev() const  { return std::sqrt(standard_variance()); }
   
double  mean() const  { return m_; }    unsigned long  count() const  { return c_; }   
double  variance() const  {return sample_variance(); }   
double  stddev() const  { return sample_stddev(); }
    double  operator ()() const  { return stddev(); }};
Then your code would use this function object with std::for_each.  It should work with anything that makes iterators, not just circular_buffer.  If you're going to take the average from several buffers, make sure to get your function object to assigned the value of the return value of for_each, so the averaging data is carried over.
Daryle W.