Polymorphic Accumulate

Hi,

Is it possible to use polymorphic classes as 'accumulators'? Consider
the situation where I want to accumulate over a vector, but the user
has the option to decide what 'function' is used. Hence, I have a Base
class (itself a binary_function) from which two classes are extended
(Add and Subtract)....

// Virtual base class defining the functor interface
class Base : public binary_function<double, double, double>
{
public:
    virtual double operator()(const double& _Left, const double& _Right)
const = 0;
};

// An adder class
class Add : public Base
{
public:
    double operator()(const double& _Left, const double& _Right) const
        {
                 return _Left + _Right;
        }
};

// A multiplier class
class Subtract : public Base
{
public:
    double operator()(const double& _Left, const double& _Right) const
        {
                 return _Left - _Right;
        }
};

..... in my code I now want to construct the relevant accumulator
function (held as a smart pointer, e.g. boost::shared_ptr) and then
call run the accumulator:

void main
{
        vector<double> a(100, 1.0); // build a vector of 1's
        vector<double>::iterator p1 = a.begin();
    vector<double>::iterator p2 = a.end();

        // ... obtain user choice ....

        // create accumulator based on user choice
        boost::shared_ptr<Base> Accumulator;
        switch (userChoice)
        {
        case ADD:
                 Accumulator = boost::shared_ptr<Base>(new Subtract);
                 break;
        case SUBTRACT:
                 Accumulator = boost::shared_ptr<Base>(new Subtract);
                 break;
        }

        // accumulate
        double result = accumulate(p1, p2, 0.0, Accumulator());
}

Obviously, this won't work, but does anyone out there know of a way of
achieving my aim? To summarise, how can I get accumulate to work with
(a) polymorphic classes, and (b) through a shared_ptr?

Many thanks,
Andy

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