
Boost Users : 
Subject: Re: [Boostusers] Troubles using boost::circular_buffer
From: Daryle Walker (darylew_at_[hidden])
Date: 20120417 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 standarddeviation 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.
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