I am looking at refactoring a lot of code and have discussed a bit in relations to the best way of handling inheritance. Given the following three classes
class Listener_Interface {
public:
virtual void message(data& data);
}
class Timing_Interface {
public:
virtual void timerEvent(data& data);
}
class Action_Interface {
public:
virtual void action(data& data);
}
There is a need for a class to implement all these plus provide a some extra specifik methods.
Should I inherit like this:
class NewClass_Interface :
public Listener_Interface,
public Timing_Interface,
public Action_Interface {
public:
virtual void newMethod();
}
class NewClass : NewClass_Interface {
....
}
or
class NewClass_Interface {
public:
virtual void newMethod();
}
class NewClass :
public NewClass_Interface
public Listener_Interface,
public Timing_Interface,
public Action_Interface {
....
}
To me the previous seems more correct and easier to test etc. But for some reason all the classes and code looks lite the latter.
It depends on your logic. Sometimes you might want your NewClass_Interface to not necessarily have a relation with Listener, Timing and Action. But since it doesn't look like the case here, I agree with you. The better you constraint the use of your interfaces, the more reliable your code will be.
So I would go with this:
class Listener_Interface {
public:
virtual void message(data& data) = 0;
}
class Timing_Interface {
public:
virtual void timerEvent(data& data) = 0;
}
class Action_Interface {
public:
virtual void action(data& data) = 0;
}
(Observe how I make your methods pure virtual in order to make your classes real interfaces)
class NewClass_Interface :
public Listener_Interface,
public Timing_Interface,
public Action_Interface
{
public:
virtual void newMethod() = 0;
}
class NewClass : NewClass_Interface {
....
}
This way you'll have better control on what is going on.
Also, I would advise you of using the most common standard for Interfaces naming: IListener, ITiming, IAction and INewClass.
Related
I have class hierarchy as shown below. It's a simplified version of actual code.
class Base
{
public :
// user_define_type is a output parameter
virtual void Fill(user_define_type);
}
class A : public Base
{
public :
void Fill(user_define_type) override;
}
class B : public Base
{
public :
void Fill(user_define_type) override;
}
I am overriding Fill() method as I need different formatting in both derived classes. Now I have to write one more class deriving from "Base" as it has common functionality. Now my problem is that new class will have to implement Fill() that will operate on different user defined type. As I am returning base class pointer from factory so new Fill() has to be virtual in base but that means I have to add it's definition in older classes "A" and "B" and throw not supported exception from them. This is not a good design. Any better design you guys can suggest ? Thanks in advance.
I believe you need to create a common base class for your user_defined_types in order to achieve this. I also think this could be a good place to use the strategy pattern.
Basically, you create
class user_defined_type_base
{
...
}
class user_defined_type_derived : public user_defined_type_base
{
...
}
class DoSomething
{
private:
DoSomethingStrategy *strategy;
public:
DoSomething(DoSomethingStrategy *strategy) { this->strategy = strategy; }
void Fill(user_defined_type_base *type) { this->strategy->Fill(type); }
}
class DoSomethingStrategy
{
public:
virtual void Fill(user_defined_type_base *obj) = 0;
}
class DoSomethingStrategyA : public DoSomethingStrategy
{
public:
void Fill(user_defined_type_base *obj)
{
...
}
}
class DoSomethingStrategyB : public DoSomethingStrategy
{
public:
void Fill(user_defined_type_base *obj)
{
...
}
}
class DoSomethingStrategyC : public DoSomethingStrategy
{
public:
void Fill(user_defined_type_base *obj)
{
...
}
}
void main()
{
DoSomethingStrategy *strategy = new DoSomethingStragegyA();
DoSomething *dosomething = new DoSomething(strategy);
user_defined_type_base *type = new user_defined_type_base();
dosomething->Fill(type);
DoSomethingStrategy *strategyC = new DoSomethingStragegyC();
DoSomething *dosomethingC = new DoSomething(strategyC);
user_defined_type_base *typeC = new user_defined_type_derived();
dosomethingC->Fill(typeC);
}
I'm asked to implement an interface and I'm wondering what would be the best strategy to factorize the code as much as possible.
Here is the interface definition (I'm not supposed to change it):
#include <string>
class BaseIf
{
public:
virtual ~BaseIf() {}
virtual std::string getName() = 0;
};
class IntIf : public BaseIf
{
public:
virtual ~IntIf() {}
virtual int getValue() = 0;
};
class FloatIf : public BaseIf
{
public:
virtual ~FloatIf() {}
virtual float getValue() = 0;
};
I'll end up with IntImpl (implementing IntIf) and FloatImpl (implementing FloatIf). But I'm wondering where I should put any code common to those two classes (like the name attribute management or any other stuff required by BaseIf which is actually much bigger than in this MCVE).
If I create BaseImpl (implementing BaseIf's getName function) with the common code, and have IntImpl derive from it (and IntIf), then I need to also implement getName in it because it's reported as not implemented. And I also get double inheritance of BaseIf...
I was wondering if Pimpl pattern would help, then IntImpl would have a BaseImpl object as attribute (and only derive from IntIf), but then, again, I need to implement getName in IntImpl to "forward" the call to the BaseImpl attribute. So as BaseIf has actually many virtual functions this is just going to be a real pain to maintain.
Is there no smart solution/pattern making it possible to implement once only getName in a common place? Or is it just the interface that is bad and should be reworked?
This is the primary use case for virtual inheritance.
Despite all the stigma that surrionds multiple and virtual inheritance, there are no particular problems when oure interfaces (no data members) are virtually inherited. Here's the gist:
class BaseIf
{
public:
virtual ~BaseIf() {}
virtual std::string getName() = 0;
};
class IntIf : public virtual BaseIf
{
public:
virtual ~IntIf() {}
virtual int getValue() = 0;
};
class BaseImpl : public virtual BaseIf
{
public:
std::string getName () override { return "whoa dude"; }
};
class IntImpl : public virtual IntIf, public BaseImpl
{
public:
int getValue() override { return 42; }
};
full demo
With a deeper hierarchy one probably would have to virtually inherit implementation classes as well, which is not very convenient but still doable.
An alternative to virtual inheritance of implementation would be to stratify the implementation into a "building blocks" layer and the final layer. Building blocks are standalone and do not inherit other building blocks. (They may inherit interfaces). The final classes inherit building blocks but not other final classes.
class BaseBlock : public virtual BaseIf
{
public:
std::string getName () override { return "whoa dude"; }
};
class IntBlock : public virtual IntIf
{
public:
int getValue() override { return 42; }
};
class BaseImpl : public BaseBlock {};
class IntImpl : public BaseBlock, public IntBlock {};
full demo
One does need to made changes to the interfaces if there was no virtual inheritance in the hierarchy. These changes are however transparent (the clients code need not be changed, only recompiled) and probably beneficial anyway.
Without virtual inheritance, one would have to resort to lots of boilerplate.
class BaseBlock // no base class!
{
public:
virtual std::string getName () { return "whoa dude"; }
};
class BaseImpl : public BaseIf, public BaseBlock
{
public:
// oops, getName would be ambiguous here, need boplerplate
std::string getName () override { return BaseBlock::getName(); }
};
You can make a template class that implements the common part of an interface like this:
template <class IFACE> class BaseImpl : public IFACE
{
public:
std::string getName () override { ... }
}
and then
class IntImpl : public BaseImpl<IntIf>
{
public:
int getValue() override { ... }
}
The result is a simple single-inheritance chain. BaseIf <- IntIf <- BaseImpl <- IntImpl
Make sure you have a good reason for IntIf and FloatIf to exist, though -- in your MCVE they look like they don't need to be there at all.
You can provide default implementation for pure virtual functions:
struct A {
virtual void frob() = 0;
};
void A::frob() {
std::cout << "default";
}
struct B : A {
void frob() override {
A::frob(); // calls the default
}
};
If I'm reading your problem correctly, you'd like a default implementation for getName(). So solve that, simply provide an implementation and call it:
class IntIf : public BaseIf
{
public:
virtual ~IntIf() {}
virtual int getValue() = 0;
std::string getName() override {
return BaseIf::getName();
}
};
class FloatIf : public BaseIf
{
public:
virtual ~FloatIf() {}
virtual float getValue() = 0;
std::string getName() override {
return BaseIf::getName();
}
};
I have two interfaces:
class FirstInterface
{
virtual int getId() const = 0;
};
class SecondInterface
{
virtual void setId(int id) = 0;
};
This is a combined interface:
class CombinedInterface : public FirstInterface, public SecondInterface
{
};
This is a concrete class of first interface:
class FirstConcrete : public FirstInterface
{
virtual int getId() const
{
return 1;
}
};
Now, this class CompleteConcrete should have the CombinedInterface but want to reuse the implementation of FirstConcrete at the same time.
class CompleteConcrete : public FirstConcrete, public SecondInterface
{
virtual void setId(int id) { }
};
// This is wrong C++
// Cannot convert from CompleteConcrete * to CombinedInterface *
// CombinedInterface * combinedInterface = new CompleteConcrete();
This is not working of course. Does anyone know a way to achieve this goal in C++ ??
Here's the virtual-inheritance based solution that I mentioned in the comments:
class FirstInterface
{
virtual int getId() const = 0;
};
class SecondInterface
{
virtual void setId(int id) = 0;
};
class CombinedInterface : virtual public FirstInterface,
virtual public SecondInterface
{
};
class FirstConcrete : virtual public FirstInterface
{
virtual int getId() const
{
return 1;
}
};
class CompleteConcrete : virtual public FirstConcrete,
virtual public CombinedInterface
{
virtual void setId(int id) { }
};
void example()
{
CombinedInterface * combinedInterface = new CompleteConcrete();
}
With virtual inheritance, the only change that's needed (besides the elephant in the room) is to have CompleteConcrete multiply-inherit from CombinedInterface, instead of SecondInterface. You can think of it this way: with CompleteConcreate in the picture, it now supports CombinedInterface, and not just the addition of SecondInterface.
Some frown on virtual inheritance. I don't. It's one of the unique features of C++, that no other high level language shared, TMK. It's a very powerful tool, and can solve certain problems that would be hard to solve in other ways. The two main disadvantages of virtual inheritance are:
Because it is so powerful, it can be easily misused, and lead to various problems.
If virtually-inherited classes have non-default constructors it quickly becomes painful, because every class that virtually inherits something is now responsible for constructing it.
But as long as virtual inheritance is used correctly, and the involved classes can take of constructing themselves, virtual inheritance is a useful tool.
P.S. I'll also mention one other alternative solution that just came to mind. If, say you have your CombinedInterface just so that it can be required for some particular function, like:
void somefunction(CombinedInterface &object);
Your function requires a combined interface.
Make a small change:
void somefunction(FirstInterface &first, SecondInterface &second);
and pass the same object as both parameters. You can pass CompleteConcrete, that implements both interfaces, without any changes to your class hierarchy. You could also have a template facade that makes it look like the function still takes one parameter:
template<typename T> void somefunction(T &&t)
{
real_somefunction(std::forward<T>(t), std::forward<T>(t));
}
void real_somefunction(FirstInterface &first, SecondInterface &second);
You can pretty much get rid of CombinedInterface, and simply pass any object that implements both interfaces to somefunction(), and your real_somefunction() will use one or the other parameter to invoke the appropriate interface.
Say you need to carry a pointer to an object that implements both interfaces?
class combined_pointer : public std::pair<FirstInterface *, SecondInterface *> {
public:
template<typename T> combined_pointer(T *t)
: std::pair<FirstInterface *, SecondInterface *>(t, t)
{}
};
Just a starting point.
I've used virtual inheritance.
This compiled successfully with a warning, but I think it is ok.
class FirstInterface
{
virtual int getId() const = 0;
};
class SecondInterface
{
virtual void setId(int id) = 0;
};
class CombinedInterface : virtual public FirstInterface, public SecondInterface
{
};
class FirstConcrete : virtual public FirstInterface
{
virtual int getId() const
{
return 1;
}
};
class CompleteConcrete : public CombinedInterface, public FirstConcrete
{
virtual void setId(int id) { }
};
// warning: C4250: inherits via dominance
CombinedInterface * combinedInterface = new CompleteConcrete();
I have interface based on another:
class IDrawable {
public:
virtual ~IDrawable();
};
class IExtendedDrawable: public IDrawable {
public:
virtual ~IExtendedDrawable();
};
class DrawableImplementation: public IDrawable {
public:
virtual ~DrawableImplementation();
};
class ExtendedDrawableImplementation:
public DrawableImplementation, public IExtendedDrawable
{
public:
virtual ~ExtendedDrawableImplementation();
};
Then ExtendedDrawableImplementation = DrawableImplementation (+IDrawable) + IExtendedDrawable (+IDrawable)
Is it right to have IDrawable twice in same class?
I'll give the benefit of the doubt that you DO indeed need/want multiple inheritance. I see it as good in only limited situations, and interfaces is one of them (even Java allows this).
As said above, use virtual inheritance and be sure to only use pure virtual methods in the interface classes.
class IDrawable {
public:
virtual ~IDrawable();
virtual void doSomething() = 0;
};
class IExtendedDrawable: virtual public IDrawable {
public:
virtual ~IExtendedDrawable();
virtual void doSomethingElse() = 0;
};
class DrawableImplementation: virtual public IDrawable {
public:
virtual ~DrawableImplementation();
virtual void doSomething() {/*code here*/}
};
class ExtendedDrawableImplementation:
public DrawableImplementation, public IExtendedDrawable
{
public:
virtual ~ExtendedDrawableImplementation();
virtual void doSomething() {/*code here*/}
virtual void doSomethingElse() {/*code here*/}
};
It largely depends on the logic inside the classes is, but IMO you're better off using composition instead:
class ExtendedDrawableImplementation : public IExtendedDrawable
{
IDrawable* drawableImplementation; //points to a DrawableImplementation
public:
virtual ~ExtendedDrawableImplementation();
};
Multiple inheritance is rarely the answer.
Just derive ExtendedDrawableImplementation from IExtendedDrawable. That way it will have both interfaces anyway. I don't see why you would want to derive from both here.
I'm not C++ programmer, but from what I remember, it's correct. You don't have IDrawable twice, but you have multiple path to it. See http://en.wikipedia.org/wiki/Diamond_problem#The_diamond_problem
There are two base classes have same function name. I want to inherit both of them, and over ride each method differently. How can I do that with separate declaration and definition (instead of defining in the class definition)?
#include <cstdio>
class Interface1{
public:
virtual void Name() = 0;
};
class Interface2
{
public:
virtual void Name() = 0;
};
class RealClass: public Interface1, public Interface2
{
public:
virtual void Interface1::Name()
{
printf("Interface1 OK?\n");
}
virtual void Interface2::Name()
{
printf("Interface2 OK?\n");
}
};
int main()
{
Interface1 *p = new RealClass();
p->Name();
Interface2 *q = reinterpret_cast<RealClass*>(p);
q->Name();
}
I failed to move the definition out in VC8. I found the Microsoft Specific Keyword __interface can do this job successfully, code below:
#include <cstdio>
__interface Interface1{
virtual void Name() = 0;
};
__interface Interface2
{
virtual void Name() = 0;
};
class RealClass: public Interface1,
public Interface2
{
public:
virtual void Interface1::Name();
virtual void Interface2::Name();
};
void RealClass::Interface1::Name()
{
printf("Interface1 OK?\n");
}
void RealClass::Interface2::Name()
{
printf("Interface2 OK?\n");
}
int main()
{
Interface1 *p = new RealClass();
p->Name();
Interface2 *q = reinterpret_cast<RealClass*>(p);
q->Name();
}
but is there another way to do this something more general that will work in other compilers?
This problem doesn't come up very often. The solution I'm familiar with was designed by Doug McIlroy and appears in Bjarne Stroustrup's books (presented in both Design & Evolution of C++ section 12.8 and The C++ Programming Language section 25.6). According to the discussion in Design & Evolution, there was a proposal to handle this specific case elegantly, but it was rejected because "such name clashes were unlikely to become common enough to warrant a separate language feature," and "not likely to become everyday work for novices."
Not only do you need to call Name() through pointers to base classes, you need a way to say which Name() you want when operating on the derived class. The solution adds some indirection:
class Interface1{
public:
virtual void Name() = 0;
};
class Interface2{
public:
virtual void Name() = 0;
};
class Interface1_helper : public Interface1{
public:
virtual void I1_Name() = 0;
void Name() override
{
I1_Name();
}
};
class Interface2_helper : public Interface2{
public:
virtual void I2_Name() = 0;
void Name() override
{
I2_Name();
}
};
class RealClass: public Interface1_helper, public Interface2_helper{
public:
void I1_Name() override
{
printf("Interface1 OK?\n");
}
void I2_Name() override
{
printf("Interface2 OK?\n");
}
};
int main()
{
RealClass rc;
Interface1* i1 = &rc;
Interface2* i2 = &rc;
i1->Name();
i2->Name();
rc.I1_Name();
rc.I2_Name();
}
Not pretty, but the decision was it's not needed often.
You cannot override them separately, you must override both at once:
struct Interface1 {
virtual void Name() = 0;
};
struct Interface2 {
virtual void Name() = 0;
};
struct RealClass : Interface1, Interface2 {
virtual void Name();
};
// and move it out of the class definition just like any other method:
void RealClass::Name() {
printf("Interface1 OK?\n");
printf("Interface2 OK?\n");
}
You can simulate individual overriding with intermediate base classes:
struct RealClass1 : Interface1 {
virtual void Name() {
printf("Interface1 OK?\n");
}
};
struct RealClass2 : Interface2 {
virtual void Name() {
printf("Interface2 OK?\n");
}
};
struct RealClass : RealClass1, RealClass2 {
virtual void Name() {
// you must still decide what to do here, which is likely calling both:
RealClass1::Name();
RealClass2::Name();
// or doing something else entirely
// but note: this is the function which will be called in all cases
// of *virtual dispatch* (for instances of this class), as it is the
// final overrider, the above separate definition is merely
// code-organization convenience
}
};
Additionally, you're using reinterpret_cast incorrectly, you should have:
int main() {
RealClass rc; // no need for dynamic allocation in this example
Interface1& one = rc;
one.Name();
Interface2& two = dynamic_cast<Interface2&>(one);
two.Name();
return 0;
}
And here's a rewrite with CRTP that might be what you want (or not):
template<class Derived>
struct RealClass1 : Interface1 {
#define self (*static_cast<Derived*>(this))
virtual void Name() {
printf("Interface1 for %s\n", self.name.c_str());
}
#undef self
};
template<class Derived>
struct RealClass2 : Interface2 {
#define self (*static_cast<Derived*>(this))
virtual void Name() {
printf("Interface2 for %s\n", self.name.c_str());
}
#undef self
};
struct RealClass : RealClass1<RealClass>, RealClass2<RealClass> {
std::string name;
RealClass() : name("real code would have members you need to access") {}
};
But note that here you cannot call Name on a RealClass now (with virtual dispatch, e.g. rc.Name()), you must first select a base. The self macro is an easy way to clean up CRTP casts (usually member access is much more common in the CRTP base), but it can be improved. There's a brief discussion of virtual dispatch in one of my other answers, but surely a better one around if someone has a link.
I've had to do something like this in the past, though in my case I needed to inherit from one interface twice and be able to differentiate between calls made on each of them, I used a template shim to help me...
Something like this:
template<class id>
class InterfaceHelper : public MyInterface
{
public :
virtual void Name()
{
Name(id);
}
virtual void Name(
const size_t id) = 0;
}
You then derive from InterfaceHelper twice rather than from MyInterface twice and you specify a different id for each base class. You can then hand out two interfaces independently by casting to the correct InterfaceHelper.
You could do something slightly more complex;
class InterfaceHelperBase
{
public :
virtual void Name(
const size_t id) = 0;
}
class InterfaceHelper1 : public MyInterface, protected InterfaceHelperBase
{
public :
using InterfaceHelperBase::Name;
virtual void Name()
{
Name(1);
}
}
class InterfaceHelper2 : public MyInterface, protected InterfaceHelperBase
{
public :
using InterfaceHelperBase::Name;
virtual void Name()
{
Name(2);
}
}
class MyClass : public InterfaceHelper1, public InterfaceHelper2
{
public :
virtual void Name(
const size_t id)
{
if (id == 1)
{
printf("Interface 1 OK?");
}
else if (id == 2)
{
printf("Interface 2 OK?");
}
}
}
Note that the above hasn't seen a compiler...
class BaseX
{
public:
virtual void fun()
{
cout << "BaseX::fun\n";
}
};
class BaseY
{
public:
virtual void fun()
{
cout << "BaseY::fun\n";
}
};
class DerivedX : protected BaseX
{
public:
virtual void funX()
{
BaseX::fun();
}
};
class DerivedY : protected BaseY
{
public:
virtual void funY()
{
BaseY::fun();
}
};
class DerivedXY : public DerivedX, public DerivedY
{
};
There are two other related questions asking nearly (but not completely) identical things:
Picking from inherited shared method names. If you want to have rc.name() call ic1->name() or ic2->name().
Overriding shared method names from (templated) base classes. This has simpler syntax and less code that your accepted solution, but does not allow for access to the functions from the derived class. More or less, unless you need to be able to call name_i1() from an rc, you don't need to use things like InterfaceHelper.