From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 21 Jan 2010 20:13:56 CST

Message-ID:

<4b58a760$0$9750$6e1ede2f@read.cnntp.org>

Hi folks,

yes, templates and virtual member functions do not fit, I know. However,

I have here a situation where exactly that would be practical:

I have N worker classes that all provide the same interface, say:

class Interface {

public:

virtual double measure(const int *data1,const int *data2) = 0;

};

class A1 : public Interface {

// implements measure...

};

class A2 : public Interface {

// implements measure...

};

A clear application of virtual functions. Now, however, the same

function ("measure") is also required for a limited set of scalar types

that is known in advance. Say, "int", "short", "long" (doesn't matter

here). The implementation of "measure" is in all implementing interfaces

similar enough to share the code and generate it by a template.

Unfortunately, while it is know for which types I will need an

implementation for, the specific type required is not known at the type

an instance of the class is created, i.e. I cannot just template the

class itself by the type. It is known at call-time, but not at creation

time.

That is, I would prefer to write:

class Interface {

public:

template <typename T>

virtual double measure(const T *data1,const T* data2) = 0;

};

with classes implementing the interface requiring to implement the

template for all types T from a set of types specified in advance.

Clearly, the above is not possible in C++ (how would I tell the compiler

which T's are part of the interface specifications.

Which alternatives exist to generate a class hierarchy similar to the

above, i.e. require implementing classes of an interface to build

templates for a given set of types?

yes, templates and virtual member functions do not fit, I know. However,

I have here a situation where exactly that would be practical:

I have N worker classes that all provide the same interface, say:

class Interface {

public:

virtual double measure(const int *data1,const int *data2) = 0;

};

class A1 : public Interface {

// implements measure...

};

class A2 : public Interface {

// implements measure...

};

A clear application of virtual functions. Now, however, the same

function ("measure") is also required for a limited set of scalar types

that is known in advance. Say, "int", "short", "long" (doesn't matter

here). The implementation of "measure" is in all implementing interfaces

similar enough to share the code and generate it by a template.

Unfortunately, while it is know for which types I will need an

implementation for, the specific type required is not known at the type

an instance of the class is created, i.e. I cannot just template the

class itself by the type. It is known at call-time, but not at creation

time.

That is, I would prefer to write:

class Interface {

public:

template <typename T>

virtual double measure(const T *data1,const T* data2) = 0;

};

with classes implementing the interface requiring to implement the

template for all types T from a set of types specified in advance.

Clearly, the above is not possible in C++ (how would I tell the compiler

which T's are part of the interface specifications.

Which alternatives exist to generate a class hierarchy similar to the

above, i.e. require implementing classes of an interface to build

templates for a given set of types?

You can use boost::mpl to generate interfaces and implementations for a

set of types like this:

[max@truth test]$ cat test.cc

#include <stdio.h>

#include <boost/mpl/vector.hpp>

#include <boost/mpl/inherit_linearly.hpp>

namespace m = boost::mpl;

typedef m::vector<short, int, long> Types;

template<class Base, class T>

struct InterfaceFunction : Base

{

using Base::measure; // unhide Base::measure

virtual double measure(const T*, const T*) = 0;

};

template<class T>

struct InterfaceFunction<m::empty_base, T>

{

// m::empty_base does not have measure()

virtual double measure(const T*, const T*) = 0;

};

// generate an interface with one measure() overload for each of the Types

typedef m::inherit_linearly<Types, InterfaceFunction<m::_1, m::_2>

::type Interface;

// a function that invokes different overloads of Interface::measure

void fun(Interface& itf)

{

short a1 = 1, b1 = 2;

itf.measure(&a1, &b1);

int a2 = 3, b2 = 4;

itf.measure(&a2, &b2);

long a3 = 3, b3 = 4;

itf.measure(&a3, &b3);

}

// these implement Inteface::measure() by forwarding it to

Derived::doMeasure()

template<class Derived, class Base, class T>

struct InterfaceFunctionImp : Base

{

using Base::measure; // unhide Base::measure

double measure(const T* t1, const T* t2)

{

return static_cast<Derived*>(this)->doMeasure(t1, t2);

}

};

template<class Derived, class T>

struct InterfaceFunctionImp<Derived, m::empty_base, T> : Interface

{

// m::empty_base does not have measure()

double measure(const T* t1, const T* t2)

{

return static_cast<Derived*>(this)->doMeasure(t1, t2);

}

};

struct A1 : m::inherit_linearly<Types, InterfaceFunctionImp<A1, m::_1,

m::_2> >::type

{

template<class T>

double doMeasure(T const* t1, T const* t2)

{

return printf("%s\n", __PRETTY_FUNCTION__);

}

};

struct A2 : m::inherit_linearly<Types, InterfaceFunctionImp<A2, m::_1,

m::_2> >::type

{

template<class T>

double doMeasure(T const* t1, T const* t2)

{

return printf("%s\n", __PRETTY_FUNCTION__);

}

};

int main(int, char** argv)

{

A1 a1;

fun(a1);

A2 a2;

fun(a2);

}

[max@truth test]$ g++ -Wall -o test test.cc

[max@truth test]$ ./test

double A1::doMeasure(const T*, const T*) [with T = short int]

double A1::doMeasure(const T*, const T*) [with T = int]

double A1::doMeasure(const T*, const T*) [with T = long int]

double A2::doMeasure(const T*, const T*) [with T = short int]

double A2::doMeasure(const T*, const T*) [with T = int]

double A2::doMeasure(const T*, const T*) [with T = long int]

--

Max

[ See http://www.gotw.ca/resources/clcm.htm for info about ]

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