Re: Polymorphic Accumulate
helix schrieb:
Is it possible to use polymorphic classes as 'accumulators'?
It is possible via the NVI ("non-virtual interface")-like ansatz,
or pimpl if you prefer, v.i.
// 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)
Never use this naming scheme! Names with preceeding underscore
and upper case letters are reserved for the implementation, see
17.4.3.1.2 in Our Holy standard. You might get quite curious effects
on different systems.
const = 0;
};
Add this non-polymorphic functor:
struct BaseWrapper {
explicit BaseWrapper(Base& b) : b(b) {}
double operator()(double left, double right) const {
return b(left, right);
}
private:
Base& b;
};
// An adder class
class Add : public Base
{
public:
double operator()(const double& _Left, const double& _Right) const
Same issue concerning naming scheme!
{
return _Left + _Right;
}
};
// A multiplier class
class Subtract : public Base
{
public:
double operator()(const double& _Left, const double& _Right) const
Same issue concerning naming scheme!
{
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
int 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());
Just replace this very last line by:
double result = accumulate(p1, p2, 0.0, BaseWrapper(*Accumulator));
Greetings from Bremen,
Daniel Kr|gler
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