How to achieve "virtual template function" in C++ - c++

Question

first off: I have read and I know now that a virtual template member function is not (yet?) possible in C++. A workaround would be to make the class a template and then use the template-argument also in the member-function.
But in the context of OOP, I find that the below example would not be very "natural" if the class was actually a template. Please note that the code is actually not working, but the gcc-4.3.4 reports: error: templates may not be ‘virtual’
#include <iostream>
#include <vector>
class Animal {
public:
template< class AMOUNT >
virtual void eat( AMOUNT amount ) const {
std::cout << "I eat like a generic Animal." << std::endl;
}
virtual ~Animal() {
}
};
class Wolf : public Animal {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a wolf!" << std::endl;
}
virtual ~Wolf() {
}
};
class Fish : public Animal {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a fish!" << std::endl;
}
virtual ~Fish() {
}
};
class GoldFish : public Fish {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a goldfish!" << std::endl;
}
virtual ~GoldFish() {
}
};
class OtherAnimal : public Animal {
virtual ~OtherAnimal() {
}
};
int main() {
std::vector<Animal*> animals;
animals.push_back(new Animal());
animals.push_back(new Wolf());
animals.push_back(new Fish());
animals.push_back(new GoldFish());
animals.push_back(new OtherAnimal());
for (std::vector<Animal*>::const_iterator it = animals.begin(); it != animals.end(); ++it) {
(*it)->eat();
delete *it;
}
return 0;
}
So creating a "Fish< Amount > foo" is kind of strange. However, it seems desirable to me to provide an arbitrary amount of food to eat for each animal.
Thus, I am searching a solution about how to achieve something like
Fish bar;
bar.eat( SomeAmount food );
This becomes particularly useful when looking at the for-loop. One might like to feed a specific amount (FoodAmount) to all of the different animals (via eat() and bind1st() e.g.), it could not be done that easily, although I wound find this very inuitive (and thus to some extent "natural). While some might want to argue now that this is due to the "uniform"-character of a vector, I think/wish that it should be possible to achieve this and I really would like to know how, as this is puzzling me for quite some time now...
[EDIT]
To perhaps clarify the motivation behind my question, I want to program an Exporter-class and let different, more specialized classes derive from it. While the top-level Exporter-class is generally only for cosmetic/structural purpose, a GraphExporter-class is derived, that should again serve as a base-class for even more specialzed export. However, similar to the Animal-example, I would like to be able to define GraphExporter* even on specialized/derived classes (e.g. on SpecialGraphExplorer) but when calling "write( out_file )", it should call the appropriate member function for SpecialGraphExporter instead of GraphExporter::write( out_file).
Maybe this makes my situation and intentions clearer.
Best,
Shadow

Answer 1

After some thinking I recognized this as the classic multi-method requirement, i.e. a method that dispatches based on the runtime type of more than one parameter. Usual virtual functions are single dispatch in comparison (and they dispatch on the type of this only).
Refer to the following:
Andrei Alexandrescu has written (the seminal bits for C++?) on implementing multi-methods using generics in 'Modern C++ design'
Chapter 11: "Multimethods" - it implements basic multi-methods, making them logarithmic (using ordered typelists) and then going all the way to constant-time multi-methods. Quite powerful stuff !
A codeproject article that seems to have just such an implementation:
no use of type casts of any kind (dynamic, static, reinterpret, const or C-style)
no use of RTTI;
no use of preprocessor;
strong type safety;
separate compilation;
constant time of multimethod execution;
no dynamic memory allocation (via new or malloc) during multimethod call;
no use of nonstandard libraries;
only standard C++ features is used.
C++ Open Method Compiler, Peter Pirkelbauer, Yuriy Solodkyy, and Bjarne Stroustrup
The Loki Library has A MultipleDispatcher
Wikipedia has quite a nice simple write-up with examples on Multiple Dispatch in C++.
Here is the 'simple' approach from the wikipedia article for reference (the less simple approach scales better for larger number of derived types):
// Example using run time type comparison via dynamic_cast
struct Thing {
virtual void collideWith(Thing& other) = 0;
}
struct Asteroid : Thing {
void collideWith(Thing& other) {
// dynamic_cast to a pointer type returns NULL if the cast fails
// (dynamic_cast to a reference type would throw an exception on failure)
if (Asteroid* asteroid = dynamic_cast<Asteroid*>(&other)) {
// handle Asteroid-Asteroid collision
} else if (Spaceship* spaceship = dynamic_cast<Spaceship*>(&other)) {
// handle Asteroid-Spaceship collision
} else {
// default collision handling here
}
}
}
struct Spaceship : Thing {
void collideWith(Thing& other) {
if (Asteroid* asteroid = dynamic_cast<Asteroid*>(&other)) {
// handle Spaceship-Asteroid collision
} else if (Spaceship* spaceship = dynamic_cast<Spaceship*>(&other)) {
// handle Spaceship-Spaceship collision
} else {
// default collision handling here
}
}
}

Answer 2

Obviously, virtual member function templates are not allowed and could not be realized even theoretically. To build a base class' virtual table, there needs to be a finite number of virtual function-pointer entries. A function template would admit an indefinite amount of "overloads" (i.e. instantiations).
Theoretically-speaking, a language (like C++) could allow virtual member function templates if it had some mechanism to specify the actual (finite) list of instantiations. C++ does have that mechanism (i.e. explicit template instantiations), so I guess it could be possible to do this in a newer C++ standard (although I have no idea what trouble it would entail for compiler vendors to implement this feature). But, that's just a theoretical discussion, in practice, this is simply not allowed. The fact remains, you have to make the number of virtual functions finite (no templates allowed).
Of course, that doesn't mean that class template cannot have virtual functions, nor does it mean that virtual functions cannot call function templates. So, there are many solutions in that vein (like the Visitor pattern or other schemes).
One solution that, I think, serves your purpose (although it is hard to comprehend) elegantly is the following (which is basically a visitor pattern):
#include <iostream>
#include <vector>
struct Eater {
virtual void operator()(int amount) const = 0;
virtual void operator()(double amount) const = 0;
};
template <typename EaterType>
struct Eater_impl : Eater {
EaterType& data;
Eater_impl(EaterType& aData) : data(aData) { };
virtual void operator()(int amount) const { data.eat_impl(amount); };
virtual void operator()(double amount) const { data.eat_impl(amount); };
};
class Animal {
protected:
Animal(Eater& aEat) : eat(aEat) { };
public:
Eater& eat;
virtual ~Animal() { delete &eat; };
};
class Wolf : public Animal {
private:
template< class AMOUNT >
void eat_impl( AMOUNT amount) const {
std::cout << "I eat like a wolf!" << std::endl;
}
public:
friend struct Eater_impl<Wolf>;
Wolf() : Animal(*(new Eater_impl<Wolf>(*this))) { };
virtual ~Wolf() { };
};
class Fish : public Animal {
private:
template< class AMOUNT >
void eat_impl( AMOUNT amount) const {
std::cout << "I eat like a fish!" << std::endl;
}
public:
friend struct Eater_impl<Fish>;
Fish() : Animal(*(new Eater_impl<Fish>(*this))) { };
virtual ~Fish() { };
};
int main() {
std::vector<Animal*> animals;
animals.push_back(new Wolf());
animals.push_back(new Fish());
for (std::vector<Animal*>::const_iterator it = animals.begin(); it != animals.end(); ++it) {
(*it)->eat(int(0));
(*it)->eat(double(0.0));
delete *it;
};
return 0;
};
The above is a neat solution because it allows you to define a finite number of overloads that you want in one place only (in the Eater_impl class template) and all you need in the derived class is a function template (and possibly additional overloads, for special cases). There is, of course, a bit of overhead, but I guess that a bit more thought could be put into it to reduce the overhead (additional reference storage and dynamic allocation of Eater_impl). I guess, the curiously recurring template pattern could probably be employed somehow to this end.

Answer 3

I think the visitor pattern can be a solution.
UPDATE
I finished my example:
#include <iostream>
#include <vector>
#include <boost/shared_ptr.hpp>
class Animal;
class Wolf;
class Fish;
class Visitor
{
public:
virtual void visit(const Animal& p_animal) const = 0;
virtual void visit(const Wolf& p_animal) const = 0;
virtual void visit(const Fish& p_animal) const = 0;
};
template<class AMOUNT>
class AmountVisitor : public Visitor
{
public:
AmountVisitor(AMOUNT p_amount) : m_amount(p_amount) {}
virtual void visit(const Animal& p_animal) const
{
std::cout << "I eat like a generic Animal." << std::endl;
}
virtual void visit(const Wolf& p_animal) const
{
std::cout << "I eat like a wolf!" << std::endl;
}
virtual void visit(const Fish& p_animal) const
{
std::cout << "I eat like a fish!" << std::endl;
}
AMOUNT m_amount;
};
class Animal {
public:
virtual void Accept(const Visitor& p_visitor) const
{
p_visitor.visit(*this);
}
virtual ~Animal() {
}
};
class Wolf : public Animal {
public:
virtual void Accept(const Visitor& p_visitor) const
{
p_visitor.visit(*this);
}
};
class Fish : public Animal {
public:
virtual void Accept(const Visitor& p_visitor) const
{
p_visitor.visit(*this);
}
};
int main()
{
typedef boost::shared_ptr<Animal> TAnimal;
std::vector<TAnimal> animals;
animals.push_back(TAnimal(new Animal()));
animals.push_back(TAnimal(new Wolf()));
animals.push_back(TAnimal(new Fish()));
AmountVisitor<int> amount(10);
for (std::vector<TAnimal>::const_iterator it = animals.begin(); it != animals.end(); ++it) {
(*it)->Accept(amount);
}
return 0;
}
this prints:
I eat like a generic Animal.
I eat like a wolf!
I eat like a fish!

Answer 4

Per Mikael's post, I have made another offshoot, using the CRTP and following Eigen's style of using derived() for an explicit subclass reference:
// Adaptation of Visitor Pattern / CRTP from:
// http://stackoverflow.com/a/5872633/170413
#include <iostream>
using std::cout;
using std::endl;
class Base {
public:
virtual void tpl(int x) = 0;
virtual void tpl(double x) = 0;
};
// Generics for display
template<typename T>
struct trait {
static inline const char* name() { return "T"; }
};
template<>
struct trait<int> {
static inline const char* name() { return "int"; }
};
template<>
struct trait<double> {
static inline const char* name() { return "double"; }
};
// Use CRTP for dispatch
// Also specify base type to allow for multiple generations
template<typename BaseType, typename DerivedType>
class BaseImpl : public BaseType {
public:
void tpl(int x) override {
derived()->tpl_impl(x);
}
void tpl(double x) override {
derived()->tpl_impl(x);
}
private:
// Eigen-style
inline DerivedType* derived() {
return static_cast<DerivedType*>(this);
}
inline const DerivedType* derived() const {
return static_cast<const DerivedType*>(this);
}
};
// Have Child extend indirectly from Base
class Child : public BaseImpl<Base, Child> {
protected:
friend class BaseImpl<Base, Child>;
template<typename T>
void tpl_impl(T x) {
cout << "Child::tpl_impl<" << trait<T>::name() << ">(" << x << ")" << endl;
}
};
// Have SubChild extend indirectly from Child
class SubChild : public BaseImpl<Child, SubChild> {
protected:
friend class BaseImpl<Child, SubChild>;
template<typename T>
void tpl_impl(T x) {
cout << "SubChild::tpl_impl<" << trait<T>::name() << ">(" << x << ")" << endl;
}
};
template<typename BaseType>
void example(BaseType *p) {
p->tpl(2);
p->tpl(3.0);
}
int main() {
Child c;
SubChild sc;
// Polymorphism works for Base as base type
example<Base>(&c);
example<Base>(&sc);
// Polymorphism works for Child as base type
example<Child>(&sc);
return 0;
}
Output:
Child::tpl_impl<int>(2)
Child::tpl_impl<double>(3)
SubChild::tpl_impl<int>(2)
SubChild::tpl_impl<double>(3)
SubChild::tpl_impl<int>(2)
SubChild::tpl_impl<double>(3)
This snippet may be found in source here: repro:c808ef0:cpp_quick/virtual_template.cc

Answer 5

Virtual template function is not allowed. However you can use one OR the other here.
You could make an interface using virtual methods and implement your various animals in terms of having an eating interface. (i.e. PIMPL)
Less human intuitive would be having a non-member non-friend template function as a free function which could take templated const reference to any animal and make them eat accordingly.
For the record you don't need templates here. Pure virtual abstract method on the base class is enough to force and interface where all animals must eat and define how they do so with an override, providing a regular virtual would be enough to say all animals can eat but if they don't have a specific way then they can use this default way.

Answer 6

You can create a template class with virtual function, and implement the function in the derived class without using template in the follwing way:
a.h:
template <class T>
class A
{
public:
A() { qDebug() << "a"; }
virtual A* Func(T _template) { return new A;}
};
b.h:
class B : public A<int>
{
public:
B();
virtual A* Func(int _template) { return new B;}
};
and the function CTOR and call
A<int>* a1=new B;
int x=1;
a1->Func(x);
unfortunately i havn't found a way to create a virtual function with template parameters without declaring the class as a template and it template type on the dervied class

Answer 7

I have copied your code and modified it, so now it should work exactly as you want:
#include <iostream>
#include <vector>
//defined new enum type
enum AnimalEnum
{
animal,
wolf,
fish,
goldfish,
other
};
//forward declarations
class Wolf;
class Fish;
class GoldFish;
class OtherAnimal;
class Animal {
private:
AnimalEnum who_really_am_I;
void* animal_ptr;
public:
//declared new constructors overloads for each type of animal
Animal(const Animal&);
Animal(const Wolf&);
Animal(const Fish&);
Animal(const GoldFish&);
Animal(const OtherAnimal&);
template< class AMOUNT >
/*removed the virtual keyword*/ void eat( AMOUNT amount ) const {
switch (this->who_really_am_I)
{
case AnimalEnum::other: //You defined OtherAnimal so that it doesn't override the eat action, so it will uses it's Animal's eat
case AnimalEnum::animal: std::cout << "I eat like a generic Animal." << std::endl; break;
case AnimalEnum::wolf: ((Wolf*)this->animal_ptr)->eat(amount); break;
case AnimalEnum::fish: ((Fish*)this->animal_ptr)->eat(amount); break;
case AnimalEnum::goldfish: ((GoldFish*)this->animal_ptr)->eat(amount) break;
}
}
void DeleteMemory() { delete this->animal_ptr; }
virtual ~Animal() {
//there you can choose if whether or not to delete "animal_ptr" here if you want or not
}
};
class Wolf : public Animal {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a wolf!" << std::endl;
}
virtual ~Wolf() {
}
};
class Fish : public Animal {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a fish!" << std::endl;
}
virtual ~Fish() {
}
};
class GoldFish : public Fish {
public:
template< class AMOUNT >
void eat( AMOUNT amount) const {
std::cout << "I eat like a goldfish!" << std::endl;
}
virtual ~GoldFish() {
}
};
class OtherAnimal : public Animal {
//OtherAnimal constructors must be defined here as Animal's constructors
OtherAnimal(const Animal& a) : Animal(a) {}
OtherAnimal(const Wolf& w) : Animal(w) {}
OtherAnimal(const Fish& f) : Animal(f) {}
OtherAnimal(const GoldFish& g) : Animal(g) {}
OtherAnimal(const OtherAnimal& o) : Animal(o) {}
virtual ~OtherAnimal() {
}
};
//OtherAnimal will be useful only if it has it's own actions and members, because if not, typedef Animal OtherAnimal or using OtherAnimal = Animal can be used, and it can be removed from above declarations and below definitions
//Here are the definitions of Animal constructors that were declared above/before:
Animal::Animal(const Animal& a) : who_really_am_I(AnimalEnum::animal), animal_ptr(nullptr) {}
Animal::Animal(const Wolf& w) : who_really_am_I(AnimalEnum::wolf), animal_ptr(new Wolf(w)) {}
Animal::Animal(const Fish& f) : who_really_am_I(AnimalEnum::fish), animal_ptr(new Fish(f)) {}
Animal::Animal(const GoldFish& g) : who_really_am_I(AnimalEnum::goldfish), animal_ptr(new GoldFish(g)) {}
Animal::Animal(const OtherAnimal& o) :
who_really_am_I(AnimalEnum::other), animal_ptr(new OtherAnimal(o)) {}
int main() {
std::vector<Animal> animals;
animals.push_back(Animal());
animals.push_back(Wolf()); //Wolf is converted to Animal via constructor
animals.push_back(Fish()); //Fish is converted to Animal via constructor
animals.push_back(GoldFish()); //GoldFish is converted to Animal via constructor
animals.push_back(OtherAnimal()); //OtherAnimal is converted to Animal via constructor
for (std::vector<Animal>::const_iterator it = animals.begin(); it != animals.end(); ++it) {
it->eat(); //this is Animal's eat that invokes other animals eat
//delete *it; Now it should be:
it->DeleteMemory();
}
animals.clear(); //All animals have been killed, and we don't want full vector of dead animals.
return 0;
}

Answer 8

In you scenario, you are trying to mix compile time polymorphism with runtime polymorphism, but it cannot be done in this "direction".
Essential, your AMOUNT template argument represents an expected interface for the type to implement based on the union of all the operations each implementation of eat uses. If you where to create an abstract type that declared each of those operations making them virtual where needed, then you could call eat with different types (that derived from your AMOUNT interface). And it would behave as expected.

Answer 9

I don't work with templates, but I think:
(1) You cannot use templates inside a class, templates are more like global types or global variables.
(2) In O.O.P., the same problem you present, and that you are trying to solve by using templates, can be solved by using inheritance.
Classes work similar to templates, you can extended by adding new things, or replace things of classes with pointers, pointers to objects (A.K.A. "references") and overriding virtual functions.
#include <iostream>
struct Animal {
virtual void eat(int amount ) {
std::cout << "I eat like a generic Animal." << std::endl;
}
virtual ~Animal() { }
};
#if 0
// example 1
struct Wolf : Animal {
virtual void eat(int amount) {
std::cout << "I eat like a wolf!" << std::endl;
}
};
struct Fish : Animal {
virtual void eat(int amount) {
std::cout << "I eat like a fish!" << std::endl;
}
};
#else
// example 2
struct AnimalFood {
virtual int readAmount() { return 5; }
virtual void showName() {
std::cout << "I'm generic animal food" << std::endl;
}
};
struct PredatorFood : AnimalFood {
virtual int readAmount() { return 500; }
virtual void showName() {
std::cout << "I'm food for a predator" << std::endl;
}
};
struct Fish : Animal {
virtual void eat(AnimalFood* aFood) {
if (aFood->readAmount() < 50) {
std::cout << "OK food, vitamines: " << aFood->readAmount() << std::endl;
} else {
std::cout << "too much food, vitamines: " << aFood->readAmount() << std::endl;
}
}
};
struct Shark : Fish {
virtual void eat(AnimalFood* aFood) {
if (aFood->readAmount() < 250) {
std::cout << "too litle food for a shark, Im very hungry, vitamines: " << aFood->readAmount() << std::endl;
} else {
std::cout << "OK, vitamines: " << aFood->readAmount() << std::endl;
}
}
};
struct Wolf : Fish {
virtual void eat(AnimalFood* aFood) {
if (aFood->readAmount() < 150) {
std::cout << "too litle food for a wolf, Im very hungry, vitamines: " << aFood->readAmount() << std::endl;
} else {
std::cout << "OK, vitamines: " << aFood->readAmount() << std::endl;
}
}
};
#endif
int main() {
// find animals
Wolf* loneWolf = new Wolf();
Fish* goldenFish = new Fish();
Shark* sharky = new Shark();
// prepare food
AnimalFood* genericFood = new AnimalFood();
PredatorFood* bigAnimalFood = new PredatorFood();
// give food to animals
loneWolf->eat(genericFood);
loneWolf->eat(bigAnimalFood);
goldenFish->eat(genericFood);
goldenFish->eat(bigAnimalFood);
sharky->eat(genericFood);
sharky->eat(bigAnimalFood);
delete bigAnimalFood;
delete genericFood;
delete sharky;
delete goldenFish;
delete loneWolf;
}
Cheers.