reset boost accumulator c++

问题

Having not found a 'boost' way of resetting an accumulator in C++, I came across a piece of code that seems to reset a boost accumulator. But don't understand how it is achieving it. The code is as below -

#include <iostream>
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics/stats.hpp>
#include <boost/accumulators/statistics/mean.hpp>
using namespace boost::accumulators;

template< typename DEFAULT_INITIALIZABLE >
inline void clear( DEFAULT_INITIALIZABLE& object )
{
        object.DEFAULT_INITIALIZABLE::~DEFAULT_INITIALIZABLE() ;
        ::new ( boost::addressof(object) ) DEFAULT_INITIALIZABLE() ;
}

int main()
{
    // Define an accumulator set for calculating the mean 
    accumulator_set<double, stats<tag::mean> > acc;

    float tmp = 1.2;
    // push in some data ...
    acc(tmp);
    acc(2.3);
    acc(3.4);
    acc(4.5);

    // Display the results ...
    std::cout << "Mean:   " << mean(acc) << std::endl;
    // clear the accumulator
    clear(acc);
    std::cout << "Mean:   " << mean(acc) << std::endl;
    // push new elements again
    acc(1.2);
    acc(2.3);
    acc(3.4);
    acc(4.5);
    std::cout << "Mean:   " << mean(acc) << std::endl;

    return 0;
}

what do lines 7 to 12 do ? How does 'clear' manage to reset the accumulator ? Also, is there a standard boost way that I am missing and any other ways of achieving what the code above has done.

回答1:

To re-initialize an object just do:

acc = {};

What it does is {} creates a default-initialized temporary object which gets assigned to acc.

回答2:

what do lines 7 to 12 do?

They call the object's destructor, and then default-construct a new object (of the same type) in the same storage.

For sensible types, this will have the same effect as what Maxim's answer suggests, which is assigning a default-constructed temporary to the existing object.

A third alternative is to use a distinct object

#include <iostream>
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics/stats.hpp>
#include <boost/accumulators/statistics/mean.hpp>
using namespace boost::accumulators;

int main()
{
    {
        // Define an accumulator set for calculating the mean 
        accumulator_set<double, stats<tag::mean> > acc;

        float tmp = 1.2;
        // push in some data ...
        acc(tmp);
        acc(2.3);
        acc(3.4);
        acc(4.5);

        // Display the results ...
        std::cout << "Mean:   " << mean(acc) << std::endl;
    } // acc's lifetime ends here, afterward it doesn't exist

    {
        // Define another accumulator set for calculating the mean
        accumulator_set<double, stats<tag::mean> > acc;

        // Display an empty result
        std::cout << "Mean:   " << mean(acc) << std::endl;

        // push elements again
        acc(1.2);
        acc(2.3);
        acc(3.4);
        acc(4.5);
        std::cout << "Mean:   " << mean(acc) << std::endl;
    }

    return 0;
}

来源：https://stackoverflow.com/questions/54572646/reset-boost-accumulator-c

标签

c++

boost

boost-accumulators