double funcA( Arraysim& buf ){  // для сырого массива другая функция
    double avg=0;
    for(int i; i<buf.length(); i++)
        avg+=buf[i];
    return avg/buf.length();
}

#include <iostream>

template <class T>
struct ISubscript
{
    virtual T & operator []( size_t ) = 0;
    virtual const T & operator []( size_t ) const = 0;
};

class Arraysim : public ISubscript<double>
{
public:
    double & operator []( size_t n ) override
    {
        return a[n];
    }

    const double & operator []( size_t n ) const override
    {
        return a[n];
    }

    size_t length() const { return N; }

private:
    enum { N = 10 };
    double a[N];
};

double funcA( const Arraysim &buf )
{
    double avg = 0;

    for ( size_t i = 0; i < buf.length(); i++ ) avg += buf[i];

    return avg / buf.length();
}

int main()
{
    Arraysim a;

    for ( size_t i = 0; i < a.length(); i++ ) a[i] = i + i / 10.0;

    std::cout << funcA( a ) << std::endl;
}

4.95