I am trying to understand the internals of https://github.com/vshymanskyy/TinyGSM/tree/master/src and am confused with how the classes are constructed.
In particular I see that in TinyGsmClientBG96.h they define a class that inherits from multiple templated parent classes.
class TinyGsmBG96 : public TinyGsmModem<TinyGsmBG96>,
public TinyGsmGPRS<TinyGsmBG96>,
public TinyGsmTCP<TinyGsmBG96, TINY_GSM_MUX_COUNT>,
public TinyGsmCalling<TinyGsmBG96>,
public TinyGsmSMS<TinyGsmBG96>,
public TinyGsmTime<TinyGsmBG96>,
public TinyGsmGPS<TinyGsmBG96>,
public TinyGsmBattery<TinyGsmBG96>,
public TinyGsmTemperature<TinyGsmBG96>
Fair enough. If I look at one of these, for example TinyGsmTemperature, I find some confusing code.
It looks like the static cast is in place so the we can call the hardware agnostic interface getTemperature() and use the implementation defined in TinyGsmBG96.
Why not use function overriding in this case?
What is the thinking behind this implementation?
Is this a common pattern in c++?
template <class modemType>
class TinyGsmTemperature
{
public:
/*
* Temperature functions
*/
float getTemperature()
{
return thisModem().getTemperatureImpl();
}
/*
* CRTP Helper
*/
protected:
inline const modemType &thisModem() const
{
return static_cast<const modemType &>(*this);
}
inline modemType &thisModem()
{
return static_cast<modemType &>(*this);
}
float getTemperatureImpl() TINY_GSM_ATTR_NOT_IMPLEMENTED;
};
Is this a common pattern in c++?
Yes, it is called CRTP - curiously recurring template pattern.
Why not use function overriding in this case?
override relies on virtual tables, causing extra runtime overhead.
What is the thinking behind this implementation?
Say, we want a class hierarchy with overridable methods. The classic OOP approach is virtual functions. However, they aren't zero-cost: when you have
void foo(Animal& pet) { pet.make_noise(); }
you don't statically know (in general) which implementation has been passed to foo() because you've erased its type from Dog (or Cat? or something else?) to Animal. So, the OOP approach uses virtual tables to find the right function at runtime.
How do we avoid this? We can instead remember the derived type statically:
template<typename Derived /* here's where we keep the type */> struct Animal {
void make_noise() {
// we statically know we're a Derived - no runtime dispatch!
static_cast<Derived&>(*this).make_noise();
}
};
struct Dog: public Animal<Dog /* here's how we "remember" the type */> {
void make_noise() { std::cout << "Woof!"; }
};
Now, let's rewrite foo() in a zero-cost manner:
template<typename Derived> void foo(Animal<Derived>& pet) { pet.make_noise(); }
Unlike the first attempt, we haven't erased the type from ??? to Animal: we know Animal<Derived> is actually a Derived, which is a templated - therefore, fully known to the compiler - type. This turns the virtual function call into a direct one (so, even allows inlining).
Related
I need to store a container of pointers to objects.
These objects have some common methods/attributes (interface) that I want to enforce (possibly at compile time) and use.
Example:
struct A{
void fly(){}
};
struct B{
void fly(){}
};
A a;
B b;
std::vector<some *> objects;
objects.push_back(&a);
objects.push_back(&b);
for(auto & el: objects)
el->fly();
The simpler solution would be A and B inherit a common base class like FlyingClass:
struct FlyingClass{
void fly(){}
};
struct A: public FlyingClass { ...
struct B: public FlyingClass { ...
and create a
std::vector<FlyingClass *> objects;
This will work and also enforce the fact that I can only add to objects things that can fly (implement FlyingClass).
But what if I need to implement some other common methods/attributes WITHOUT coupling them with the above base class?
Example:
struct A{
void fly(){}
void swim(){}
};
struct B{
void fly(){}
void swim(){}
};
And i would like to do:
for(auto & el: objects) {
el->fly();
...
el->swim();
...
}
More in general i would be able to call a function passing one of these pointers and access both the common methods/attributes, like:
void dostuff(Element * el){
el->fly();
el->swim();
}
I could try to inherit from another interface like:
struct SwimmingClass{
void swim(){}
};
struct A: public FlyingClass, public SwimmingClass { ...
struct B: public FlyingClass, public SwimmingClass { ...
But then what the container should contain?
std::vector<FlyingClass&&SwimmingClass *> objects;
Sure, i could implement SwimmingFlyingClass, but what if i need RunningClass etc.. This is going to be a nightmare.
In other words, how can I implement a pointer to multiple interfaces without coupling them?
Or there is some template way of rethinking the problem?
Even run time type information could be acceptable in my application, if there is an elegant and maintainable way of doing this.
It is possible to do this, in a pretty TMP-heavy way that's a little expensive at runtime. A redesign is favourable so that this is not required. The long and short is that what you want to do isn't possible cleanly without language support, which C++ does not offer.
As for the ugly, shield your eyes from this:
struct AnyBase { virtual ~AnyBase() {} }; // All derived classes inherit from.
template<typename... T> class Limited {
AnyBase* object;
template<typename U> Limited(U* p) {
static_assert(all<is_base_of<T, U>...>::value, "Must derive from all of the interfaces.");
object = p;
}
template<typename U> U* get() {
static_assert(any<is_same<U, T>...>::value, "U must be one of the interfaces.");
return dynamic_cast<U*>(object);
}
}
Some of this stuff isn't defined as Standard so I'll just run through it. The static_assert on the constructor enforces that U inherits from all of T. I may have U and T the wrong way round, and the definition of all is left to the reader.
The getter simply requires that U is one of the template arguments T.... Then we know in advance that the dynamic_cast will succeed, because we checked the constraint statically.
It's ugly, but it should work. So consider
std::vector<Limited<Flying, Swimming>> objects;
for(auto&& obj : objects) {
obj.get<Flying>()->fly();
obj.get<Swimming>()->swim();
}
You are asking for something which doesn't make sense in general, that's why there is no easy way to do it.
You are asking to be able to store heterogeneus objects in a collection, with interfaces that are even different.
How are you going to iterate over the collections without knowing the type? You are restricted to the least specific or forced to do dynamic_cast pointers and cross fingers.
class Entity { }
class SwimmingEntity : public Entity {
virtual void swim() = 0;
}
class FlyingEntity : public Entity {
virtual void fly() = 0;
}
class Fish : public SwimmingEntity {
void swim() override { }
}
class Bird : public FlyingEntity {
void fly() override { }
}
std:vector<Entity*> entities;
This is legal but doesn't give you any information to the capabilities of the runtime Entity instance. It won't lead anywhere unless you work them out with dynamic_cast and rtti (or manual rtti) so where's the advantage?
This is pretty much a textbook example calling for type erasure.
The idea is to define an internal abstract (pure virtual) interface class that captures the common behavior(s) you want, then to use a templated constructor to create a proxy object derived from that interface:
#include <iostream>
#include <vector>
#include <memory>
using std::cout;
struct Bird {
void fly() { cout << "Bird flies\n"; }
void swim(){ cout << "Bird swims\n"; }
};
struct Pig {
void fly() { cout << "Pig flies!\n"; }
void swim() { cout << "Pig swims\n"; }
};
struct FlyingSwimmingThing {
// Pure virtual interface that knows how to fly() and how to swim(),
// but does not depend on type of underlying object.
struct InternalInterface {
virtual void fly() = 0;
virtual void swim() = 0;
virtual ~InternalInterface() { }
};
// Proxy inherits from interface; forwards to underlying object.
// Template class allows proxy type to depend on object type.
template<typename T>
struct InternalImplementation : public InternalInterface {
InternalImplementation(T &obj) : obj_(obj) { }
void fly() { obj_.fly(); }
void swim() { obj_.swim(); }
virtual ~InternalImplementation() { }
private:
T &obj_;
};
// Templated constructor
template<typename T>
FlyingSwimmingThing(T &obj) : proxy_(new InternalImplementation<T>(obj))
{ }
// Forward calls to underlying object via virtual interface.
void fly() { proxy_->fly(); }
void swim() { proxy_->swim(); }
private:
std::unique_ptr<InternalInterface> proxy_;
};
int main(int argc, char *argv[])
{
Bird a;
Pig b;
std::vector<FlyingSwimmingThing> objects;
objects.push_back(FlyingSwimmingThing(a));
objects.push_back(FlyingSwimmingThing(b));
objects[0].fly();
objects[1].fly();
objects[0].swim();
objects[1].swim();
}
The same trick is used for the deleter in a shared_ptr and for std::function. The latter is arguably the poster child for the technique.
You will always find a call to "new" in there somewhere. Also, if you want your wrapper class to hold a copy of the underlying object rather than a pointer, you will find you need a clone() function in the abstract interface class (whose implementation will also call new). So these things can get very non-performant very easily, depending on what you are doing...
[Update]
Just to make my assumptions clear, since some people appear not to have read the question...
You have multiple classes implementing fly() and swim() functions, but that is all that the classes have in common; they do not inherit from any common interface classes.
The goal is to have a wrapper object that can store a pointer to any one of those classes, and through which you can invoke the fly() and swim() functions without knowing the wrapped type at the call site. (Take the time to read the question to see examples; e.g. search for dostuff.) This property is called "encapsulation"; that is, the wrapper exposes the fly() and swim() interfaces directly and it can hide any properties of the wrapped object that are not relevant.
Finally, it should be possible to create a new otherwise-unrelated class with its own fly() and swim() functions and have the wrapper hold a pointer to that class (a) without modifying the wrapper class and (b) without touching any call to fly() or swim() via the wrapper.
These are, as I said, textbook features of type erasure. I did not invent the idiom, but I do recognize when it is called for.
I have an expensive function defined in a base class, which depends on low level information from its derived classes:
class BaseClass{
...
// Defined in derived class
virtual int low_level(int)=0;
// Expensive function depending on the pure virtual function
void myExpensiveFunction(){
for(...){
for(...){
for(...){
... = low_level(...);
...
}
}
}
}
};
class DerivedClass : public BaseClass{
// A very cheap operation that can be inlined:
inline virtual int low_level(int i){
return a[i];
}
// Calling the base class function
void test(){
myExpensiveFunction();
}
};
If I understand things correctly, the fact that the low-level function is virtual prevents it from being inlined in the code above. Now, I was thinking about a way to get around this and thought of the following solution, where I pass a pointer to the derived class member function as a template parameter:
class BaseClass{
...
// The function is now templated by the derived function:
template<typename D, int (D::*low_level)(int)>
void myExpensiveFunction(){
for(...){
for(...){
for(...){
... = static_cast<D*>(this)->low_level(...);
...
}
}
}
}
};
class DerivedClass : public BaseClass{
// A very cheap operation that can be inlined:
inline int low_level(int i){
return a[i];
}
// Calling the base class function
void test(){
myExpensiveFunction<DerivedClass,&DerivedClass::low_level>();
}
};
Does this strategy make sense? I imagine that the low level operation will be inlined when the expensive base class function is expanded in the derived class.
I tested implementing it and it compiles and works, but I haven't seen any noticeable differences in performance.
Kind regards,
Joel
Passing the function you want to call using a pointer to member to a base class doesn't really improve over using a virtual function. In fact, I would expect it to make the situation worse. An alternative approach is to use a function object with an inline function call operator and call this. The "normal" approach is to kind of invert the class hierarchy and use the Curiously Recurring Template Pattern: the idea is to create a template which will derive from its template argument. The template argument is expected to provide the customization points, e.g. the function low_level.
Depending on the situation, you could also try to avoid inheritance altogether and do something like this instead:
template<typename LL>
class HighLevel {
LL lowLevel;
public:
HighLevel(LL const &ll) : lowLevel(ll) { }
void myExpensiveFunction() {
for(...) {
for(...) {
for(...) {
... = lowLevel.low_level(...);
...
}
}
}
}
};
class LowLevel {
public:
inline int low_level(int i) { // note: not virtual
return a[i];
}
};
Used like:
HighLevel<LowLevel> hl;
hl.myExpensiveFunction();
If you don't want different types of HighLevel<...> objects floating around, you could derive all those from an abstract, non-template class HighLevelBase which exposes a virtual void myExpensiveFunction() = 0 that gets implemented in the template.
Whether or not this makes sense for your situation, I cannot tell without more information, but I find that C++ often offers better tools than inheritance to solve particular problems.
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.
So in structure like
struct RenderCastedDataFunctor
{
simpleRendererGraphElement* obj_;
RenderCastedDataFunctor(simpleRendererGraphElement* obj)
: obj_(obj) { }
void operator()(char* castedChar, int castedCharLength)
{
obj_->renderCastedData(castedChar, castedCharLength);
}
};
can we turn simpleRendererGraphElement* into abstract type and make its function name we use in structure (renderCastedData) abstract too?
So I have a function inside charGenerator class
template <typename Function>
void AddSubscriberToGeneratedData(Function f)
I want to pass to it functions from different classes of type void (differentClass::*)(char*, int)
With that structure inside some simpleRendererGraphElement I can subscribe function called renderCastedData to data with
charGenerator->AddSubscriberToGeneratedData(RenderCastedDataFunctor(this));
I want to have a way to be capable to pass abstract class function that takes char* and int to AddSubscriberToGeneratedData. How to do such thing?
can we turn
simpleRendererGraphElement* into
abstract type and make its function
name we use in structure
(renderCastedData) abstract too?
Very very good idea. You should do this. Make the class abstract by making it's functions virtual, and then define a concrete class (deriving from this abstract class) which implements the virtual functions. That would be a better design!
And the rest seems already fine. You don't have to do anything, as you're doing this:
AddSubscriberToGeneratedData(RenderCastedDataFunctor(this));
I suppose, here this represents the pointer to an instance of the concrete class. If so, then that should work!
EDIT:
I understand how good this Idea is but
I do not get how to implement it. that
is why I am asking.
Alright. Here is an example:
class AbstractGraphElement
{
public:
virtual void RenderCastedData(char* castedChar, int castedCharLength) = 0;
};
This is your abstract class, and RenderCastedData is a pure virtual function. Now you need to define a concrete class which must define RenderCastedData function. So here it is:
class SimpleGraphElement : public AbstractGraphElement
{
public:
virtual void RenderCastedData(char* castedChar, int castedCharLength)
{
//function body - define it yourself
}
};
Done!
Now what you need to do is this. Modify RenderCastedDataFunctor as follows:
struct RenderCastedDataFunctor
{
AbstractGraphElement* m_graphElement;
RenderCastedDataFunctor(AbstractGraphElement* graphElement)
: m_graphElement(graphElement) { }
void operator()(char* castedChar, int castedCharLength)
{
m_graphElement->RenderCastedData(castedChar, castedCharLength);
}
};
Then add subscriber,
AbstractGraphElement *pGraphElement = new SimpleGraphElement();
AddSubscriberToGeneratedData(RenderCastedDataFunctor(pGraphElement));
I think it gave your some idea, right? The important point is : use pointer of type AbstractGraphElement but initialize this pointer with SimpleGraphElement. I think, you should read about virtual functions, and runtime polymorphism. That would help you a lot.
There was an interesting problem in C++, but it was more about architecture.
There are many (10, 20, 40, etc) classes describing some characteristics (mix-in classes), for example:
struct Base { virtual ~Base() {} };
struct A : virtual public Base { int size; };
struct B : virtual public Base { float x, y; };
struct C : virtual public Base { bool some_bool_state; };
struct D : virtual public Base { string str; }
// ....
The primary module declares and exports a function (for simplicity just function declarations without classes):
// .h file
void operate(Base *pBase);
// .cpp file
void operate(Base *pBase)
{
// ....
}
Any other module can have code like this:
#include "mixing.h"
#include "primary.h"
class obj1_t : public A, public C, public D {};
class obj2_t : public B, public D {};
// ...
void Pass()
{
obj1_t obj1;
obj2_t obj2;
operate(&obj1);
operate(&obj2);
}
The question is how do you know what the real type of a given object in operate() is without using dynamic_cast and any type information in classes (constants, etc)? The operate() function is used with a big array of objects in small time periods and dynamic_cast is too slow for it and I don't want to include constants (enum obj_type { ... }) because this is not the OOP-way.
// module operate.cpp
void some_operate(Base *pBase)
{
processA(pBase);
processB(pBase);
}
void processA(A *pA)
{
}
void processB(B *pB)
{
}
I cannot directly pass a pBase to these functions. And it's impossible to have all possible combinations of classes, because I can add new classes just by including new header files.
One solution that came to mind, in the editor I can use a composite container:
struct CompositeObject
{
vector<Base *pBase> parts;
};
But the editor does not need time optimization and can use dynamic_cast for parts to determine the exact type. In operate() I cannot use this solution.
So, is it possible to avoid using a dynamic_cast and type information to solve this problem? Or maybe I should use another architecture?
The real problem here is about what you are trying to achieve.
Do you want something like:
void operate(A-B& ) { operateA(); operateB(); }
// OR
void operate(A-B& ) { operateAB(); }
That is, do you want to apply an operation on each subcomponent (independently), or do you wish to be able to apply operations depending on the combination of components (much harder).
I'll take the first approach here.
1. Virtual ?
class Base { public: virtual void operate() = 0; };
class A: virtual public Base { public virtual void operate() = 0; };
void A::operate() { ++size; } // yes, it's possible to define a pure virtual
class obj1_t: public A, public B
{
public:
virtual void operate() { A::operate(); B::operate(); }
};
Some more work, for sure. Notably I don't like the repetition much. But that's one call to the _vtable, so it should be one of the fastest solution!
2. Composite Pattern
That would probably be the more natural thing here.
Note that you can perfectly use a template version of the pattern in C++!
template <class T1, class T2, class T3>
class BaseT: public Base, private T1, private T2, private T3
{
public:
void operate() { T1::operate(); T2::operate(); T3::operate(); }
};
class obj1_t: public BaseT<A,B,C> {};
Advantages:
no more need to repeat yourself! write operate once and for all (baring variadic...)
only 1 virtual call, no more virtual inheritance, so even more efficient that before
A, B and C can be of arbitrary type, they should not inherit from Base at all
edit the operate method of A, B and C may be inlined now that it's not virtual
Disadvantage:
Some more work on the framework if you don't have access to variadic templates yet, but it's feasible within a couple dozen of lines.
First thing that comes to mind is asking what you really want to achieve... but then again the second thought is that you can use the visitor pattern. Runtime type information will implicitly be used to determine at what point in the hierarchy is the final overrider of the accept method, but you will not explicitly use that information (your code will not show any dynamic_cast, type_info, constants...)
Then again, my first thought comes back... since you are asking about the appropriateness of the architecture, what is it that you really want to achieve? --without knowledge of the problem you will only find generic answers as this one.
The usual object oriented way would be to have (pure) virtual functions in the base class that are called in operate() and that get overridden in the derived classes to execute code specific to that derived class.
Your problem is that you want to decide what to do based on more than one object's type. Virtual functions do this for one object (the one left of the . or ->) only. Doing so for more than one object is called multiple dispatch (for two objects it's also called double dispatch), and in C++ there's no built-in feature to deal with this.
Look at double dispatch, especially as done in the visitor pattern.