10. 立方

#include <iostream>
double cube(double a);
double refcube(const double &ra);
int main()
{
    using namespace std;
    double x = 3.0;

    cout << cube(x);
    cout << " = cube of " << x << endl;
    cout << refcube(x + 1);
    cout << " = cube of " << x + 1 << endl;
}

double cube(double a)
{
    a *= a * a;
    return a;
}

double refcube(const double &ra)
{
    return ra * ra * ra;
}