I would like to implement the following thing in C++:
I would like to have a bunch of child classes of a single class with the ability to call a function that takes a pair of objects of any of these types. There is supposed to be a generic implementation that is called for mixed types or the base type and specialised implementations which get called if two objects of the same derived type are used as arguments.
As far as I know, this is a classic application of double dispatch. However, I have the following constraint:
It must be possible to derive new classes from the existing ones and add new pair-functions for these new classes without changing existing classes, for instance in an external library..
The approach I proposed in my last question is faulty, and the solution proposed there only works for types that are known at the time when the base class is written.
Any suggestion on how to implement this? Is that even possible?
Update: Code says more than a thousand words. The following approach works:
#include <iostream>
class B;
class A
{
public:
virtual void PostCompose(A* other)
{
other->PreCompose(this);
}
virtual void PreCompose(A* other)
{
std::cout << "Precomposing with an A object" << std::endl;
}
virtual void PreCompose(B* other);
};
class B : public A
{
public:
using A::PreCompose;
virtual void PostCompose(A* other)
{
other->PreCompose(this);
}
virtual void PostCompose(B* other)
{
other->PreCompose(this);
}
virtual void PreCompose(B* other)
{
std::cout << "Precomposing with a B object" << std::endl;
}
};
void A::PreCompose(B* other)
{
PreCompose((A*)other);
}
int main()
{
B b;
A* p = &b;
p->PostCompose(p); // -> "Precomposing with a B object"
}
but it requires knowledge of B when implementing A. Is there a better way?
Since the derived classes only need to detect if the parameter type matches the object type, you can just use a straightforward check.
virtual void foo( base *argument_base ) {
if ( derived *argument = dynamic_cast< derived * >( argument_base ) ) {
argument->something = pair_match_foo;
} else {
base_class::foo( argument_base );
}
}
Related
Is there a way to write an abstract base class that looks like it's forcing an implementer to choose among a myriad of pure virtual functions?
The abstract base classes I'm writing define a mathematically tedious function, and request that the deriving code define only building block functions. The building block functions can be generalized to take on more arguments, though. For example, in the code below, it might "make sense" to allow another_derived::first() to take three arguments. The "mathematically tedious" part of this is the multiplication by 3. Unsurprisingly, it won't allow won't compile unless I comment out the creation of d2. I understand why.
One option is to create different base classes. One would request a single parameter function to be defined, and the other would request a two parameter function to be defined. However, there would be an enormous amount of code being copy and pasted between the two base class' definition of final_result(). This is why I'm asking, so I don't write WET code.
Another option would be to have one pure virtual function, but change the signature so that its implementation can do either of these things. I want to explore this, but I also don't want to start using fancier techniques so that it puts a barrier to entry on the type of people trying to inherit from these base classes. Ideally, if the writers of the base class could get away with barely knowing any c++, that would be great. Also, it would be ideal if the inheritors didn't even have to know about the existence of related classes they could be writing.
#include <iostream>
class base{
public:
virtual int first(int a) = 0;
int final_result(int a) {
return 3*first(a);
}
};
class derived : public base {
public:
int first(int a) {
return 2*a;
}
};
class another_derived : public base {
public:
int first(int a, int b) {
return a + b;
}
};
int main() {
derived d;
std::cout << d.final_result(1) << "\n";
//another_derived d2; // doesn't work
return 0;
}
Not sure it matches exactly what you want, but with CRTP, you might do something like:
template <typename Derived>
struct MulBy3
{
template <typename... Ts>
int final_result(Ts... args) { return 3 * static_cast<Derived&>(*this).first(args...); }
};
class derived : public MulBy3<derived> {
public:
int first(int a) { return 2*a; }
};
class another_derived : public MulBy3<another_derived > {
public:
int first(int a, int b) { return a + b; }
};
With usage similar to
int main() {
derived d;
std::cout << d.final_result(1) << "\n";
another_derived d2;
std::cout << d2.final_result(10, 4) << "\n";
}
Demo
I have a hierarchy of classes:
class Base
{
public:
Base():a{5}{}
virtual ~Base(){};
int a;
};
class Derived : public Base
{
public:
Derived():b{10}{}
int b;
};
I then have a class template that operates on whatever type it is instanciated with:
template<typename T>
class DoStuff
{
public:
DoStuff():val{}{}
virtual ~DoStuff(){};
virtual void printDoStuff() = 0;
T getVal(){return val;};
private:
T val;
};
class DoStuffWithInt : public DoStuff<int>
{
public:
virtual void printDoStuff() override {cout << "val = " << getVal() << endl;}
};
class DoStuffWithBase : public DoStuff<Base>
{
public:
virtual void printDoStuff() {cout << "a = " << getVal().a << endl;}
};
Now I would like to have a hierarchy of class like this:
class DoStuffWithBase : public DoStuff<Base>
{
public:
virtual void printDoStuff() {printVal(); cout << "a = " << getVal().a << endl;}
};
// Wrong and will not compile, trying to make a point
class DoStuffWithDerived : public DoStuffWithBase<Derived>
{
public:
void printDoStuff() override {DoStuffWithBase::printDoStuff(); cout << "b = " << getVal().b << endl;}
};
Basically I would like to have DoStuffWithBase that operates on a base be extended so that I can reuse its functions, but the extended class DoStuffWithDerived should operate on a Derived type.
I managed to get something working by templating DoStuffWithBase with a pointer to Base and extending it:
template <class T>
static void deleteIfPointer(const T& t)
{
std::cout << "not pointer" << std::endl;
}
template <class T>
static void deleteIfPointer(T* t)
// ^
{
std::cout << "is pointer" << std::endl;
delete t;
}
template<typename T>
class DoStuff
{
public:
DoStuff():val{}{}
DoStuff(const T& value):val{value}{};
virtual ~DoStuff(){deleteIfPointer(val);}
virtual void printDoStuff() = 0;
T getVal(){return val;};
private:
T val;
};
class DoStuffWithBase : public DoStuff<Base*>
{
public:
// New base
DoStuffWithBase(): DoStuff(new Base()){}
DoStuffWithBase(Base* b) : DoStuff(b){}
virtual void printDoStuff() {printVal(); cout << "a = " << getVal()->a << endl;}
};
class DoStuffWithDerived : public DoStuffWithBase
{
public:
// New derived
DoStuffWithDerived(): DoStuffWithBase(new Derived()){}
void printDoStuff() override {DoStuffWithBase::printDoStuff(); cout << "b = " << static_cast<Derived*>(getVal())->b << endl;}
};
It works but there are several things I don't like:
The code is a lot more complicated, when 99% of the time, I won't need to extend a DoStuffWithX class, I will just use DoStuffWithInt, DoStuffWithClass, DoStuffWithAnotherClass etc... Here I had to add several constructors, a special case destructor and so on.
I have to use pointers and manage them (static_cast when needed, deletion...), all in order to avoid slicing and get the right type. Also, DoStuff::val should theorically not be null, but with a pointer there is no way I can prevent that (or atleast I don't know one). Maybe using smart pointers would help a bit here ? I am not super familiar with them.
I have to manage cases where T is a pointer and when it is not. For example, the deleteIfPointer function above, but also switching between . and -> and probably more.
Is there any simpler way to achieve what I am trying to do ? A design pattern or something else ? Am I stuck with my solution and is it somewhat good ?
Edit: I tried to implement it with std::variant as in #Tiger4Hire's answer:
class Derived : public Base
{
public:
Derived():b{10}{}
int b;
};
class Derived2 : public Base
{
public:
Derived2():c{12}{}
int c;
};
using DerivedTypes = std::variant<Derived, Derived2>;
struct VariantVisitor
{
void operator()(Derived& d)
{
d.b = 17;
}
void operator()(Derived2& d)
{
d.c = 17;
}
};
class DoStuffWithVariant : public DoStuff<DerivedTypes>
{
public:
void handleBasePart(Base& base)
{
cout << "a = " << base.a << endl;
base.a = 10;
}
virtual void printDoStuff() override
{
auto unionVal_l = getVal();
if (std::holds_alternative<Derived>(unionVal_l))
{
std::cout << "the variant holds a Derived!\n";
auto& derived_l = std::get<0>(unionVal_l);
cout << "b = " << derived_l.b << endl;
handleBasePart(derived_l);
}
else if (std::holds_alternative<Derived2>(unionVal_l))
{
std::cout << "the variant holds a Derived2!\n";
auto& derived2_l = std::get<1>(unionVal_l);
cout << "c = " << derived2_l.c << endl;
handleBasePart(derived2_l);
}
std::visit(VariantVisitor{}, unionVal_l);
}
};
What I like about it:
I don't have to use pointers.
I feel the code is less tricky, easier to understand.
What I don't like about it:
The code is all in one place and it deals with all the possible Derived types (and even the Base type) at once whereas with inheritance, classes are more specialized, you can really look at a class and directly know what it does, what it overrides etc... On the other hand one could argue that it means the algorithm is in one place instead of dispatched all over the classes hierarchy.
You can't have an abstract base class as your interface.
All in all it is a really good alternative, but I am still wondering if there is a simpler way to implement dynamic polymorphism ? Do you necessarily have to resort to (base class) pointers with dynamic polymorphism ? Are std::variant the way to go now ?
Edit2: 2 other drawbacks with variants that I didn't notice at first:
All your derived class and your base class have to be defined in the same library. Clients can't easily add a new Derived class since it would mean modifying the variant and they might not have access to it.
On the project I am working on, base classes are defined in one library, and are derived in other independant "sub" libraries. So if I try to use variant in my main library, it won't be able to access the Derived types in the sub libraries, which is a major issue.
If your base class implenting the variant (DoStuff here) has other members, when you call std::visit on the variant, you might have to also embark the needed other members of DoStuff. I think you should be able to use lambdas to capture them, but still, it's a lot less straightforward than using them directly as in the case of inheritance.
Your core problem is that you cast away your type information.
C++ will always call the right function, if it knows the correct type. This is why the pattern of pointer-to-base is almost always an anti-pattern (even though it is often taught as the "C++" way to do things).
Modern C++-style is to hold things as strongly-typed pointers, and cast them to the base pointer object, only when calling a function that takes a base-pointer as a parameter.
The standard supports this way of working by providing std::variant. Thus rather than
std::vector<Base*> my_list_of_things;
my_list_of_things.push_back(new Derived); // casting away type is bad
You start with
using DerivedTypes = std::variant<std::unique_ptr<Derived1>,
std::unique_ptr<Derived2>/*,etc*/>;
std::vector<DerivedTypes> my_list_of_things;
Now you can iterate over the list, calling a function which takes a pointer-to-base, casting away the type information only during the call.
You can also visit the members of the list, with a function (often a lambda) that knows exactly the type it is working on.
So you get the best of both worlds!
This does assume you have access to C++17 or above though, also that you are not working with code that is a library (compiled) but allows the library user to make their own classes. For example, libraries like Qt can't use this way of working.
If you don't have access to C++17, you may find curiously recursing templates fit much of what you are doing. (This is a controversial pattern though, as it is ugly and confusing)
I'm finding it difficult to describe this problem very concisely, so I've attached the code for a demonstration program.
The general idea is that we want a set of Derived classes that are forced to implement some abstract Foo() function from a Base class. Each of the derived Foo() calls must accept a different parameter as input, but all of the parameters should also be derived from a BaseInput class.
We see two possible solutions so far, neither we're very happy with:
Remove the Foo() function from the base class and reimplement it with the correct input types in each Derived class. This, however, removes the enforcement that it be implemented in the same manner in each derived class.
Do some kind of dynamic cast inside the receiving function to verify that the type received is correct. However, this does not prevent the programmer from making an error and passing the incorrect input data type. We would like the type to be passed to the Foo() function to be compile-time correct.
Is there some sort of pattern that could enforce this kind of behaviour? Is this whole idea breaking some sort of fundamental idea underlying OOP? We'd really like to hear your input on possible solutions outside of what we've come up with.
Thanks so much!
#include <iostream>
// these inputs will be sent to our Foo function below
class BaseInput {};
class Derived1Input : public BaseInput { public: int d1Custom; };
class Derived2Input : public BaseInput { public: float d2Custom; };
class Base
{
public:
virtual void Foo(BaseInput& i) = 0;
};
class Derived1 : public Base
{
public:
// we don't know what type the input is -- do we have to try to cast to what we want
// and see if it works?
virtual void Foo(BaseInput& i) { std::cout << "I don't want to cast this..." << std::endl; }
// prefer something like this, but then it's not overriding the Base implementation
//virtual void Foo(Derived1Input& i) { std::cout << "Derived1 did something with Derived1Input..." << std::endl; }
};
class Derived2 : public Base
{
public:
// we don't know what type the input is -- do we have to try to cast to what we want
// and see if it works?
virtual void Foo(BaseInput& i) { std::cout << "I don't want to cast this..." << std::endl; }
// prefer something like this, but then it's not overriding the Base implementation
//virtual void Foo(Derived2Input& i) { std::cout << "Derived2 did something with Derived2Input..." << std::endl; }
};
int main()
{
Derived1 d1; Derived1Input d1i;
Derived2 d2; Derived2Input d2i;
// set up some dummy data
d1i.d1Custom = 1;
d2i.d2Custom = 1.f;
d1.Foo(d2i); // this compiles, but is a mistake! how can we avoid this?
// Derived1::Foo() should only accept Derived1Input, but then
// we can't declare Foo() in the Base class.
return 0;
}
Since your Derived class is-a Base class, it should never tighten the base contract preconditions: if it has to behave like a Base, it should accept BaseInput allright. This is known as the Liskov Substitution Principle.
Although you can do runtime checking of your argument, you can never achieve a fully type-safe way of doing this: your compiler may be able to match the DerivedInput when it sees a Derived object (static type), but it can not know what subtype is going to be behind a Base object...
The requirements
DerivedX should take a DerivedXInput
DerivedX::Foo should be interface-equal to DerivedY::Foo
contradict: either the Foo methods are implemented in terms of the BaseInput, and thus have identical interfaces in all derived classes, or the DerivedXInput types differ, and they cannot have the same interface.
That's, in my opinion, the problem.
This problem occured to me, too, when writing tightly coupled classes that are handled in a type-unaware framework:
class Fruit {};
class FruitTree {
virtual Fruit* pick() = 0;
};
class FruitEater {
virtual void eat( Fruit* ) = 0;
};
class Banana : public Fruit {};
class BananaTree {
virtual Banana* pick() { return new Banana; }
};
class BananaEater : public FruitEater {
void eat( Fruit* f ){
assert( dynamic_cast<Banana*>(f)!=0 );
delete f;
}
};
And a framework:
struct FruitPipeLine {
FruitTree* tree;
FruitEater* eater;
void cycle(){
eater->eat( tree->pick() );
}
};
Now this proves a design that's too easily broken: there's no part in the design that aligns the trees with the eaters:
FruitPipeLine pipe = { new BananaTree, new LemonEater }; // compiles fine
pipe.cycle(); // crash, probably.
You may improve the cohesion of the design, and remove the need for virtual dispatching, by making it a template:
template<class F> class Tree {
F* pick(); // no implementation
};
template<class F> class Eater {
void eat( F* f ){ delete f; } // default implementation is possible
};
template<class F> PipeLine {
Tree<F> tree;
Eater<F> eater;
void cycle(){ eater.eat( tree.pick() ); }
};
The implementations are really template specializations:
template<> class Tree<Banana> {
Banana* pick(){ return new Banana; }
};
...
PipeLine<Banana> pipe; // can't be wrong
pipe.cycle(); // no typechecking needed.
You might be able to use a variation of the curiously recurring template pattern.
class Base {
public:
// Stuff that don't depend on the input type.
};
template <typename Input>
class Middle : public Base {
public:
virtual void Foo(Input &i) = 0;
};
class Derived1 : public Middle<Derived1Input> {
public:
virtual void Foo(Derived1Input &i) { ... }
};
class Derived2 : public Middle<Derived2Input> {
public:
virtual void Foo(Derived2Input &i) { ... }
};
This is untested, just a shot from the hip!
If you don't mind the dynamic cast, how about this:
Class BaseInput;
class Base
{
public:
void foo(BaseInput & x) { foo_dispatch(x); };
private:
virtual void foo_dispatch(BaseInput &) = 0;
};
template <typename TInput = BaseInput> // default value to enforce nothing
class FooDistpatch : public Base
{
virtual void foo_dispatch(BaseInput & x)
{
foo_impl(dynamic_cast<TInput &>(x));
}
virtual void foo_impl(TInput &) = 0;
};
class Derived1 : public FooDispatch<Der1Input>
{
virtual void foo_impl(Der1Input & x) { /* your implementation here */ }
};
That way, you've built the dynamic type checking into the intermediate class, and your clients only ever derive from FooDispatch<DerivedInput>.
What you are talking about are covariant argument types, and that is quite an uncommon feature in a language, as it breaks your contract: You promised to accept a base_input object because you inherit from base, but you want the compiler to reject all but a small subset of base_inputs...
It is much more common for programming languages to offer the opposite: contra-variant argument types, as the derived type will not only accept everything that it is bound to accept by the contract, but also other types.
At any rate, C++ does not offer contravariance in argument types either, only covariance in the return type.
C++ has a lot of dark areas, so it's hard to say any specific thing is undoable, but going from the dark areas I do know, without a cast, this cannot be done. The virtual function specified in the base class requires the argument type to remain the same in all the children.
I am sure a cast can be used in a non-painful way though, perhaps by giving the base class an Enum 'type' member that is uniquely set by the constructor of each possible child that might possibly inherit it. Foo() can then check that 'type' and determine which type it is before doing anything, and throwing an assertion if it is surprised by something unexpected. It isn't compile time, but it's the closest a compromise I can think of, while still having the benefits of requiring a Foo() be defined.
It's certainly restricted, but you can use/simulate coviarance in constructors parameters.
I'm trying to take advantage of the polymorphism in c++, but I'm from a c world, and I think what I've done could be done more cleverly in a OOP way.
I have 2 classes that has exactly the same public attributes, and I want to "hide" that there exists 2 different implementations. Such that I can have a single class where I can use the member functions as If i were accessing the specific class.
An very simple implementation of what I'm trying to accomplish is below:
#include <iostream>
class subber{
private:
int id;
public:
int doStuff(int a,int b) {return a-b;};
};
class adder{
private:
int id;
public:
int doStuff(int a, int b) {return a+b;};
};
class wrapper{
private:
int type_m;
adder cls1;
subber cls2;
public:
wrapper(int type) {type_m=type;};//constructor
int doStuff(int a, int b) {if(type_m==0) return cls1.doStuff(a,b); else return cls2.doStuff(a,b);};
};
int main(){
wrapper class1(0);
std::cout <<class1.doStuff(1,3) <<std::endl;
wrapper class2(1);
std::cout <<class2.doStuff(1,3) <<std::endl;
return 0;
}
I have 2 classes called "subber" and "adder" which both have a member function called doStuff, which will either subtract of add 2 numbers.
This I wrap up in a class "wrapper", which has both "adder" and "subber" as private variables, and a doStuff public member function. And given which value I instantiate my "wrapper" class with, my "wrapper" class will simply relay the "doStuff" to the correct class.
This code does of cause work, but I would like to avoid instatiating both "subber" and "adder" in my wrapper class, since I will only need of them in each of my "wrapper" classes.
Thanks
There are many ways to do it. Through a Factory for example.
But to keep it simple - make a base abstract class that defines the interface, and derive your classes from it to implement the functionality. Then you only need to make the distinction once, when you create the class, after that you don't care, you just call the interface functions.
your code would look something like that.
class DoStuffer
{
public:
virtual int doStuff(int, int)=0;
virtual ~DoStuffer(){}; // Because Tony insists:-) See the comments
}
class subber: public DoStuffer{
public:
virtual int doStuff(int a,int b) {return a-b;};
};
class adder: public DoStuffer{
public:
virtual int doStuff(int a, int b) {return a+b;};
};
int main(){
DoStuffer *class1 = new adder();
DoStuffer *class2 = new subber();
std::cout <<class1->doStuff(1,3) <<std::endl;
std::cout <<class2->doStuff(1,3) <<std::endl;
delete class1; // don't forget these:-)
delete class2;
return 0;
}
This is one of the more idiomatic ways to use the C++ class system to accomplish what you want. Both adder and subber publicly inherit from wrapper, which is now an abstract base class. The doStuff method is now a (pure) virtual function. And instead of being a simple instance of wrapper, the "encapsulated" object is now a reference to a wrapper.
#include <iostream>
class wrapper {
public:
virtual int doStuff(int a, int b) = 0;
};
class subber : public wrapper {
public:
virtual int doStuff(int a,int b) {return a - b;}
};
class adder : public wrapper {
public:
virtual int doStuff(int a, int b) {return a + b;}
};
int main(){
// actual objects
adder impl1;
subber impl2;
// in real code, the wrapper references would probably be function arguments
wrapper& class1 = impl1;
std::cout << class1.doStuff(1,3) << std::endl;
wrapper& class2 = impl2;
std::cout << class2.doStuff(1,3) << std::endl;
return 0;
}
(Not using any factory pattern in this example, since it's not obvious that it's needed or what the question is about.)
Exactly what was last said.
Make a base class, and have a virtual function |doStuff| in it.
Then you can derive any number of classes out from it, all have to implement the above virtual function, in whatever way they want to.
Then you can just do the following
BaseClass *object1 = new DerivedClass1();
BaseClass *object2 = new DerivedClass2();
..
You can even do
object1 = object2;
And then they point to the same object (i.e. an object of type |DerivedClass2|)
But remember, when you do objectn->doStuff(), the function that will be executed will be what the pointer points to at run-time, and not at compile time.
i.e. if I do object1->doStuff() DerivedClass2's doStuff will be called because we already did `object1 = object2;
You may want to Google and read about
Polymorphism/ Run-time Polymorphism
Virtual Functions in C++
You can read Factory Method, which is something that is known as a Design Pattern, but later in life.
Thanks
The classic run-time polymorphic approach is:
struct Operation
{
virtual ~Operation() { } // guideline: if there are any virtual functions,
// provide virtual destructor
virtual int doStuff(int, int) const;
};
struct Subber : Operation
{
int doStuff(int a, int b) const { return a - b; }
};
struct Adder : Operation
{
int doStuff(int a, int b) const { return a + b; }
};
enum Operations { Add, Subtract };
struct Operation* op_factory(Operations op)
{
if (op == Add) return new Adder;
if (op == Subtract) return new Subber;
throw std::runtime_error("unsupported op");
}
int main()
{
Operation* p1 = op_factory(Add);
std::cout << p1->doStuff(1,3) <<std::endl;
Operation* p2 = op_factory(Subtract);
std::cout << p2->doStuff(1,3) <<std::endl;
delete p1;
delete p2;
}
From the Standard 5.3.5/5 "In the first alternative (delete object), if the static type of the operand is different from its dynamic type, the static type shall be a base class of the operand's dynamic type and the static type shall have a virtual destructor or the behavior is undefined.", which is why you must use the virtual keyword on the base class destructor.
It's noteworthy that in your example the type of operation to perform was communicated to the wrapper class using a function argument of 0 or 1... this is what suggests you want run-time polymorphism. For example, if the 0 or 1 value was based on a command line argument, file content, keyboard input etc., then the factory method above can pass a corresponding Add or Subtract value and receive an appropriately-behaving object derived from Operation. This concept of creating an instance of a run-time polymorphic type based on run-time values is known as a factory.
If you really only need compile-time polymorphism, you can do some interesting things with templates such as:
template <class Operation>
void output(int a, int b)
{
std::cout << Operation::doStuff(a, b) << std::endl;
std::cout << Operation::doStuff(a * 10, b * 10) << std::endl;
std::cout << Operation::doStuff(a * 100, b * 100) << std::endl;
}
int main()
{
output<adder>(1, 3);
output<subber>(1, 3);
}
FWIW, your approach is probably slightly faster than the virtual function approach (as it can potentially do more inlining), but not as clean, extensible, maintainable or scalable.
I think what you're looking for is virtual functions. If you declare a function virtual in your base class, you can do things like make a vector containing multiple objects derived from your base class, but when you call on a particular object it will execute it's own method.
I have the following classes :
class A {
};
class B : public A {
};
class P {
private:
std::list<A*> l
protected:
virtual void DoIt(A* a) = 0;
public:
void WorkerThread() { for (it=l.begin(); it!=l.end(); it++) DoIt(*it); }
};
class Q : public P
{
protected:
void DoIt(A* a) { print("false"); }
void DoIt(B* b) { print("true"); }
};
Unfortunately, DoIt(B* b) will never get called.
DoIt(A* a) will always be called even if I add B objects to the list.
What can I do to make DoIt(B* b) called ?
Is it possible to achieve this if B does not know Q ?
Is it possible to achieve this if without dynamic cast ?
Thank you
Well, nobody's really directly answered your question (well, heavyd tried) so I will. Some other "answers" here are actually more helpful for fixing your problem though.
The issue is that void DoIt(B*) is NOT an override of the virtual function DoIt(A*). It's an overload. There's a HUGE difference.
When you say that DoIt(B*) is not called when you pass a B* I have to assume that you're holding references or pointers to you Q through a pointer to something higher up the higherarchy. In those cases the static name resolution only finds DoIt(A*) and since B* is-a A* it gets upcasted and that's the version that gets called. Since it is virtual the override in Q is what gets called.
If you had a pointer to Q as a pointer to Q though, and called DoIt with a B* the DoIt(B*) function should get called. At this point, double dispatch is not needed and is not used.
You need double dispatch when you have two abstract types and a function that must behave differently based on the concrete types of both abstractions. This is what you're attempting to do when you call DoIt with B on Q at a higher level than static naming provides. There are too many methods that answer different needs to be able to suggest one solution over another in your case, don't really know what you're trying to solve. In fact, you might not even need it! A better approach for you might be to implement DoIt(B*) as a virtual function in the top of your higherarchy.
I would suggest that you get Andre Alexandrescu's book, Modern C++ Design, and look it over. He explains a pretty darn cool visitor implementation as well as a multiple dispatch mechanism that scales. Don't stop there though, there's other great implementations that can answer the question differently.
Good luck.
You are looking for a double dispatch mechanism that is not built into the language. There are different approaches on how this can be implemented based on the visitor pattern. Google for double-dispatch in C++. Note that this is a patch and not easily extended to big hierarchies:
struct visitor;
struct A {
virtual void accept( visitor& v ) { v(*this); }
};
struct B {
virtual void accept( visitor& v ) { v(*this); }
};
struct visitor {
virtual void operator()( A& ) = 0;
virtual void operator()( B& ) = 0;
};
struct myvisitor : visitor {
void operator( A& ) { std::cout << "A" << std::endl; }
void operator( B& ) { std::cout << "B" << std::endl; }
};
int main() {
std::vector<A*> data = ...
myvisitor v;
for ( std::vector<A*>::iterator it = data.begin(), end = data.end(); it != end; ++it )
{
(*it)->accept( v );
}
}
The usual mechanism will be used and accept will be dispatched to the final overrider of the method, which in turn will call the visitor method. Now, at that point, the static type of the argument to the visitor operator() is in fact the actual type that you want to call the function with.
DoIt(B* b) will never get called because you are never passing in objects of type B*, every time you call DoIt, at least in the given code, you are passing in objects of type A*.
Consider the situation where the override of Doit(A* a) did not exist. Your current code would not compile because it the compiler cannot implicitly cast an object of type A* to B*.
What are you expecting the behaviour to be if someone passes in an A* but the underlying type is really a B?
You might be looking for something like this:
class A
{
public:
virtual ~A() {}
virtual bool isB() const { return false; }
};
class B : public A
{
public:
bool isB() const { return true; }
};
void Q::DoIt( A* a )
{
print( a->isB() ? "true" : "false" );
}
You're looking for multiple dispatch or multimethods. Wikipedia has a nice example for c++; link here.
What you are trying to do is known as multiple dispatch and won't work in C++ because function overloading is static. Take a look at the wikipedia article for some possible work arounds.
For example, if you don't want the logic for the DoIt functionality in the A and B classes themselves as a virtual function then you could use the dynamic_cast method:
class A {
};
class B : public A {
};
class P : protected std::list<A*>
{
protected:
virtual void DoIt(A* a) = 0;
public:
void WorkerThread() { for (it=begin(); it!=end(); it++) DoIt(*it); }
};
class Q : public P
{
protected:
void DoIt(A* a) {
if(B *b = dynamic_cast<B*>(a)) {
// It's a B*, you can "use" b here
print("true");
} else {
// It's an A*
print("false");
}
}
};