From:

"James Kanze" <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 23 Jan 2007 10:41:07 CST

Message-ID:

<1169558797.300748.143590@v45g2000cwv.googlegroups.com>

Is it possible to use polymorphic classes as 'accumulators'?

It's certainly possible, using the letter/envelope idiom, but you

may not want to. std::accumulate copies the accumulator a lot,

and copying is an expensive operation with the letter/envelope

idiom. (In addition, since one of the copies ends up modified,

COW doesn't really help much.)

Consider

the situation where I want to accumulate over a vector, but the user

has the option to decide what 'function' is used.

the situation where I want to accumulate over a vector, but the user

has the option to decide what 'function' is used.

You can pass an additional parameter with the operator to

accumulate. Note that this binary function also gets copied,

but nowhere near as often as the accumulator, so the

letter/envelop idiom is quite doable for it. (And since it is

never modified, COW is also a valid option.)

Hence, I have a Base

class (itself a binary_function) from which two classes are extended

(Add and Subtract)....

class (itself a binary_function) from which two classes are extended

(Add and Subtract)....

Just a note concerning your vocabulary: the accumumator is the

object in which the results are accumulated, not the operation

being done. (The third parameter to std::accumulate, not the

fourth.)

// 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;

};

class Base : public binary_function<double, double, double>

{

public:

virtual double operator()(const double& _Left, const double& _Right)

const = 0;

};

You definitly want to add a virtual destructor to this.

// 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

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

Just for the record, it's "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());

}

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?

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?

Well, the letter/envelope idiom works fine; it can also be used

with a shared pointer instead of a raw pointer, using COW, or,

as is the case here, never deep copying, since all of the member

functions are const. Or just based on what you have here:

template< typename T >

class AbstractOperator

{

public:

virtual ~AbstractOperator() {}

virtual T operator()( T const& lhs, T const& rhs )

const = 0 ;

} ;

template< typename T >

class PolymorphicOperator

{

public:

PolymorphicOperator( AbstractOperator< T > const* operator )

: myOperator( operator )

{

}

T operator()( T const& lhs, T const& rhs ) const

{

return (*myOperator)( lhs, rhs ) ;

}

private:

AbstractOperator< T > const* myOperator ;

} ;

--

James Kanze (GABI Software) email:james.kanze@gmail.com

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