This is similar to this question and this one, but I think (hope!) different enough to deserve an explanation.
I have a complex configuration framework, with decorator classes used to implement some common, simple actions (like flagging when a class Set accessor is called). I'm trying to introduce a new decorator (rather than a composition), which itself "should" inherit from that same common "Set-flagging" decorator.
I'm running into "ambiguous conversion from derived class to base class", and my attempts to work around it have failed.
I'm missing something obvious, I'm sure.
Here's a very simple example, without all my framework stuff.
class A {
public:
template <class T> void Set(T& arg, T val);
bool Ready() const;
};
class B : private A {
// Does stuff where I want to flag Set() actions
};
class C : public B, public A {
// This class needs the B interface, and the Set()-flagging
public:
void SetParam(double val) { Set(param, val); }
private:
double param;
};
Note that I original used virtual inheritance of A, but in practice I need to keep the "Set-flag" for B distinct from the "Set-flag" for C, hence my attempt above.
The private inheritance above was my first attempt to avoid the ambiguity. I also tried introducing using directives in C:
class C : public B, public A {
// This class needs the B interface, and the Set()-flagging
using A::Set;
using A::Ready;
};
This doesn't change the error. I understand from searching that the ambiguity is caught before the public/private state is checked. But I thought that the explicit using directives would resolve it. Why doesn't it? Do I need to go in and use A::Set(...) or A::Ready() explicitly everywhere?
Two solutions.
If you really want to keep the private and multiple inheritance:
class A {
public:
void Set() {};
bool Ready() const {};
};
class B : private A {
};
class C : public B, public A {
public:
void SetParam() { C::A::Set(); }
};
Or if that is not needed, then a simpler one:
class A {
public:
void Set() {};
bool Ready() const {};
};
class B : public A {
};
class C : public B {
public:
void SetParam() { Set(); }
};
Related
I have the following setup:
class ComponentBase...
class A : public ComponentBase...
class B : public ComponentBase...
class C : public ComponentBase...
....
class ComponentOwner
{
public:
ComponentBase *b;
ComponentOwner(ComponentBase *newb) : b(newb) {};
bool couple(ComponentOwner *other)
{ //this in a non-header file, obviously
return couple(b, other->b);
};
....
}
There's overloaded functions for each combination of ComponentBase subclasses, that each return the same type and take the same sort of arguments, like this:
bool couple(A, A);
bool couple(A, B);
bool couple(B, A); //should point to couple(A,B)
....
Sadly this doesn't work, since I can't access the type of the ComponentBase pointer without knowing how to cast.
I guess my question is this: How would I go about redirecting two base pointers with unknown type to functions that are there for their type, with as little as possible extra definitions for the subclasses (like enums)?
Thanks.
EDIT 1:
It was proposed for me to add virtual functions, assuming to the ComponentBase.
class ComponentBase
{
public:
virtual bool couple(ComponentBase *other) = 0;
.....
}
class A : public ComponentBase
{
public:
bool couple(ComponentBase *other)
{
Do I understand the proposal right until here? How would I go about resolving the type of other?
I have the following diamond class structure that does not compile:
class Base{
int a;
public:
virtual void doSomething();
};
class NotMineToTouch : public Base {};
class MyParentClass : public Base {};
class EvilDiamond : public NotMineToTouch, public MyParentClass {};
// I need these methods (that I cannot necessarily edit) to work for an EvilDiamond
void processBase (Base* b) { b->doSomething; /*...*/} // Cannot edit
void processParent (MyParentClass* p) { p->doSomething; /*...*/} // Can edit
void processNotMine (NotMineToTouch* n) { n->doSomething; /*...*/} // Cannot edit
I know the normal solution is to inherit virtually from Base; however, I am not allowed to change NotMineToTouch (or Base). Is there another solution? I am allowed to change MyParentClass and EvilDiamond at my pleasure; however, EvilDiamond must inherit from MyParentClass and NotMineToTouch, and MyParentClass must inherit from Base and may not inherit from EvilDiamond.
I challenge the following assertion:
EvilDiamond must inherit from MyParentClass and NotMineToTouch
You can probably do something along these lines (depending on your architecture):
class EvilDiamond;
class NotMineToTouchImpl : public NotMineToTouch {
EvilDiamond* tgt_;
public:
NotMineToTouchImpl(EvilDiamond* tgt) : tgt_(tgt) {}
... implement NotMineToTouch here, using tgt_ where you would have used this
};
class MyParentClassImpl : public MyParentClass {
EvilDiamond* tgt_;
public:
MyParentClassImpl(EvilDiamond* tgt) : tgt_(tgt) {}
... implement Base here, using tgt_ where you would have used this
};
class EvilDiamond {
friend class NotMineToTouchImpl;
friend class MyParentClassImpl;
// Creating permanent instances of the API classes
// may or may not be appropriate in your case.
NotMineToTouchImpl nmti_;
MyParentClassImpl pci_;
public:
EvilDiamond () : nmti_(this), pci_(this) {}
NotMineToTouchImpl* getAsNotMineToTOuch() {return &nmti_;}
MyParentClassImpl * getAsParentClass() {return &pci_;}
};
You don't have diamond as you don't use virtual inheritance.
you have some "Y" inheritance currently (EvilDiamond has 2 Base).
Without changing your classes, you may add overloads to instruct compiler what to do:
void processBase (EvilDiamond* evil) {
processBase(static_cast<NotMineToTouch*>(evil)); // Use NotMineToTouch::Base
processBase(static_cast<MyParentClass*>(evil)); // Use MyParentClass::Base
}
I'm writing a C++ library for optimization, and I've encountered a curious issue with contra-variant types.
So, I define a hierarchy of "functions", based on what information they can compute.
class Function {
public:
double value()=0;
}
class DifferentiableFunction : public Function {
public:
const double* gradient()=0;
}
class TwiceDifferentiableFunction : public DifferentiableFunction {
public:
const double* hessian()=0;
}
Which is all well and good, but now I want to define interfaces for the optimizers. For example, some optimizers require gradient information, or hessian information in order to optimize, and some don't. So the types of the optimizers are contravariant to the types of the functions.
class HessianOptimizer {
public:
set_function(TwiceDifferentiableFunction* f)=0;
}
class GradientOptimizer : public HessianOptimizer {
public:
set_function(DifferentiableFunction* f)=0;
}
class Optimizer: public GradientOptimizer {
public:
set_function(TwiceDifferentiableFunction* f)=0;
}
Which I suppose makes sense from a type theoretic perspective, but the thing that is weird about it is that usually when people want to extend code, they will inherit the already existing classes. So for example, if someone else was using this library, and they wanted to create a new type of optimizer that requires more information than the hessian, they might create a class like
class ThriceDifferentiableFunction: public TwiceDifferentiableFunction }
public:
const double* thirdderivative()=0;
}
But then to create the corresponding optimizer class, we would have to make HessianOptimizer extend ThirdOrderOptimizer. But the library user would have to modify the library to do so! So while we can add on the ThriceDifferentiableFunction without having to modify the library, it seems like the contravariant types lose this property. This seems to just be an artifact of the fact the classes declare their parent types rather than their children types.
But how are you supposed to deal with this? Is there any way to do it nicely?
Since they're just interfaces, you don't have to be afraid of multiple inheritance with them. Why not make the optimiser types siblings instead of descendants?
class OptimizerBase
{
// Common stuff goes here
};
class HessianOptimizer : virtual public OptimizerBase {
public:
virtual set_function(TwiceDifferentiableFunction* f)=0;
}
class GradientOptimizer : virtual public OptimizerBase {
public:
virtual set_function(DifferentiableFunction* f)=0;
}
class Optimizer : virtual public OptimizerBase {
public:
virtual set_function(TwiceDifferentiableFunction* f)=0;
}
// impl
class MyGradientOptimizer : virtual public GradientOptimizer, virtual public HessianOptimizer
{
// ...
};
For example, when creating a class library, I would like to specify an internal API and a public API for each classes, so I can hide some details from the user. The internal API would be used by other classes in the library, and the public API would be used by the library user.
Is it possible?
In C++, interface could mean many things. It could mean pure virtual functions that you implement in the derived classes, as in the following example,
class Interface
{
public:
virtual void f() = 0 ;
};
class Implementation : public Interface
{
public:
virtual void f() {}
};
--
Or it could mean just public functions in your class:
class A
{
public:
void f() {} //public function - an interface that the outside world can
//use to talk to your class.
};
You can use either of these and can make use of access-specifiers ( public, protected, private) to make your interfaces public or internal/private!
Kind of.
Some libraries use friend classes/functions for this. Each class declares other classes as friends if they need access to more than the "public" interface:
class Car {
friend class Mechanic;
private:
Engine engine;
};
class Mechanic {
// something involving Car::engine...
};
It's not very pretty, but it works.
Another approach that might work for you is the pimpl (pointer-to-implementation) idiom:
class CarImpl; // declaration only
class Car {
private:
CarImpl *impl;
public:
CarImpl *getImpl(); // doesn't strictly belong in the pimpl pattern
// methods that simply call the corresponding methods on impl
};
The internal interface can be accessed through a getImpl() call. You would put the CarImpl declaration in a header file that is clearly marked as internal, so clients won't access it. For example, you could put such headers in a subdirectory called internal.
The obvious drawback is that the Car class has a bunch of trivial methods that you have to implement.
A third approach, that I do not recommend, is inheritance:
class Car {
public:
virtual void start() = 0;
static Car *create();
};
And in an internal header:
class CarImpl : public Car {
public:
virtual void start();
};
The Car class only exposes the public interface; to get access to the internal interface, internal code needs to do a downcast to CarImpl. This is ugly.
You can use many tricks to grant friendship or an "extended" interface to a given few, however it is soon cumbersome.
The simplest way to separate the external interface from the internal interface... is to have two interfaces, thus two classes.
If you take a peek at the set of Design Patterns proposed by the GoF, you'll notice the Proxy pattern.
You can use this by not exposing the class to the exterior of your library, but instead exposing a Proxy, in which you wrap the class, and which only exposes a subset of its interface.
class MyClass
{
public:
void foo();
void bar();
void printDebugInfo();
void test();
};
class MyClassProxy
{
public:
MyClassProxy(std::unique_ptr<MyClass> ptr): _ptr(ptr) {}
void foo() { _ptr->foo(); }
void bar() { _ptr->bar(); }
private:
std::unique_ptr<MyClass> _ptr;
};
I personally find this design rather clean. No down-casting, No subtle inheritance trick, No friendship list longer than my arm.
I'm not quite sure what you're asking, but if you have an abstract class defined:
class Loggable { ... };
You can inherit from it privately, like this:
class User : private Loggable { ... };
The class User now has the members of Loggable, but they are private.
Please see the C++ FAQ lite.
There is a number of ways to approach this. One is runtime polymorphism:
struct widget {
virtual debug_info diagnose() = 0;
virtual void draw() = 0;
};
struct window {
virtual void draw() = 0;
};
struct view : public window, public widget {
void draw();
debug_info diagnose(); // internal
};
Or with compile-time polymorphism:
struct view {
void draw();
debug_info diagnose(); // internal
};
template<class W>
void do_it(W window)
{
widget.draw();
}
template<class W>
void diagnose_it(W widget)
{
debug_info d = widget.diagnose();
}
Another approach is to expose private members to specific functions or classes:
struct widget {
virtual void draw() = 0;
};
struct view : public widget {
friend void diagnose_widget(widget w);
void draw();
private:
debug_info diagnose();
};
// internal
debug_info diagnose_widget(widget w)
{
debug_info d = w.diagnose();
}
A C++ class has 3 levels of protection: public, protected and private. Public things are accessible to everybody, protected only to descendant -- and then for themselves and not for other descendants --, private for the class and its friend.
Thus friendship is the only way to grant more than public access to a class/function which isn't a descendant, and it grants full access, which isn't always convenient.
An heavy solution which I've used with success was to write a wrapper which was a friend of the main class, and then provided additional access to its own friends (which were the only one able to construct the wrapper). I'm not really recommending it, it is tedious, but it could be useful if you have such a need.
class Main {
public:
...
private: // but wrapped
void foo();
protected:
...
private: // and not wrapped
void bar();
};
class Wrapper {
pubic:
void foo() { wrapped.foo(); }
private:
Wrapper(Main& m) : wrapped(ma) {}
Main& wrapped;
friend void qux(Main&);
};
void qux(Main& m) {
Wrapper mwrapped(m)
mwrapped.foo();
// still can't access bar
}
I have a class (class A) that is designed to be inherited by other classes written by other people.
I also have another class (class B), that also inherits from A.
B has to access some A's member functions that shouldn't be accessed by other inheriting classes.
So, these A's member functions should be public for B, but private for others.
How can I solve it without using 'friend' directive?
Thank you.
EDIT: Example why I need it.
class A
{
public:
void PublicFunc()
{
PrivateFunc();
// and other code
}
private:
virtual void PrivateFunc();
};
class B : public class A
{
private:
virtual void PrivateFunc()
{
//do something and call A's PrivateFunc
A::PrivateFunc(); // Can't, it's private!
}
};
You can't. That's what friend is for.
An alternative would be to change the design/architecture of your program. But for hints on this I'd need some more context.
What you say is: there are two sets of subclasses of A. One set should have access, the other set shouldn't. It feels wrong to have only one brand of subclasses (i.e. B) 'see' A's members.
If what you mean is: only we can use this part of functionality, while our clients can't, there are other resorts.
(Functionality reuse by inheritance often corners you with this kind of problems. If you go towards reuse by aggregation, you may get around it.)
A suggestion:
// separate the 'invisible' from the 'visible'.
class A_private_part {
protected:
int inherited_content();
public:
int public_interface();
};
class B_internal : public A_private_part {
};
class A_export : private A_private_part {
public:
int public_interface() { A_private_part::public_interface(); }
};
// client code
class ClientClass : public A_export {
};
But better would be to go the aggregation way, and split the current "A" into a visible and an invisible part:
class InvisibleFunctionality {
};
class VisibleFunctionality {
};
class B {
InvisibleFunctionality m_Invisible;
VisibleFunctionality m_Visible;
};
// client code uses VisibleFunctionality only
class ClientClass {
VisibleFunctionality m_Visible;
};
Well - if you want exactly what you've described, then friend is the best solution. Every coding standard recommends not using friend but where the alternative design is more complex - then maybe it's worth making an exception.
To solve the problem without friend will require a different architecture
One solution might be to use a form of the pImpl idiom where 'B' derives from the inner implementation object, while the other clients derive from the outer class.
Another might be to place an extra layer of inheritance between 'A' and the "other clients". Something like:
class A {
public:
void foo ();
void bar ();
};
class B : public A { // OK access to both 'foo' and 'bar'
};
class ARestricted : private A {
public:
inline void foo () { A::foo (); }; // Forwards 'foo' only
};
However, this solution still has it's problems. 'ARestricted' cannot convert to an 'A' so this would need to be solved by some other "getter" for 'A'. However, you could name this function in such a way as it cannot be called accidentally:
inline A & get_base_type_A_for_interface_usage_only () { return *this; }
After trying to think of other solutions, and assuming that your hierarchy needs to be as you describe, I recommend you just use friend!
EDIT: So xtofl suggested renaming the types 'A' to 'AInternal' and 'ARestricted' to 'A'.
That works, except I noticed that 'B' would no longer be an 'A'. However, AInternal could be inherited virtually - and then 'B' could derive from both 'AInternal' and 'A'!
class AInternal {
public:
void foo ();
void bar ();
};
class A : private virtual AInternal {
public:
inline void foo () { A::foo (); }; // Forwards 'foo' only
};
// OK access to both 'foo' and 'bar' via AInternal
class B : public virtual AInternal, public A {
public:
void useMembers ()
{
AInternal::foo ();
AInternal::bar ();
}
};
void func (A const &);
int main ()
{
A a;
func (a);
B b;
func (b);
}
Of course now you have virtual bases and multiple inheritance! Hmmm....now, is that better or worse than a single friend declaration?
I think you have a bigger problem here. Your design doesn't seem sound.
1) I think the 'friend' construct is problematic to begin with
2) if 'friend' isn't what you want, you need to re-examine your design.
I think you either need to do something that just gets the job done, using 'friend' or develop a more robust architecture. Take a look at some design patterns, I'm sure you'll find something useful.
EDIT:
After seeing your sample code, you definitely need to re-arch. Class A may not be under your control, so that's a little tricky, but maybe want you want to re-do Class B to be a "has-a" class instead of an "is-a" class.
public Class B
{
B()
{
}
void someFunc()
{
A a; //the private functions is now called and a will be deleted when it goes out of scope
}
};
I find this a interesting challenge. Here is how I would solve the problem:
class AProtectedInterface
{
public:
int m_pi1;
};
class B;
class A : private AProtectedInterface
{
public:
void GetAProtectedInterface(B& b_class);
int m_p1;
};
class B : public A
{
public:
B();
void SetAProtectedInterface(::AProtectedInterface& interface);
private:
::AProtectedInterface* m_AProtectedInterface;
};
class C : public A
{
public:
C();
};
C::C()
{
m_p1 = 0;
// m_pi1 = 0; // not accessible error
}
B::B()
{
GetAProtectedInterface(*this);
// use m_AProtectedInterface to get to restricted areas of A
m_p1 = 0;
m_AProtectedInterface->m_pi1 = 0;
}
void A::GetAProtectedInterface(B& b_class)
{
b_class.SetAProtectedInterface(*this);
}
void B::SetAProtectedInterface(::AProtectedInterface& interface)
{
m_AProtectedInterface = &interface;
}
If you where going to use this sort of pattern all the time, you could reduce the code by using templates.
template<class T, class I>
class ProtectedInterfaceAccess : public I
{
public:
void SetProtectedInterface(T& protected_interface)
{
m_ProtectedInterface = &protected_interface;
}
protected:
T& GetProtectedInterface()
{
return *m_ProtectedInterface;
}
private:
T* m_ProtectedInterface;
};
template<class T, class I>
class ProtectedInterface : private T
{
public:
void SetupProtectedInterface(I& access_class)
{
access_class.SetProtectedInterface(*this);
}
};
class Bt;
class At : public ProtectedInterface <::AProtectedInterface, Bt>
{
public:
int m_p1;
};
class Bt : public ProtectedInterfaceAccess<::AProtectedInterface, At>
{
public:
Bt();
};
class Ct : public At
{
public:
Ct();
};
Ct::Ct()
{
m_p1 = 0;
// m_pi1 = 0; // not accessible error
}
Bt::Bt()
{
SetupProtectedInterface(*this);
m_p1 = 0;
GetProtectedInterface().m_pi1 = 0;
}
If I understand:
A will be subclassed by other developers.
B will be subclassed by other developers and inherits from A.
A has some methods you don't want accessible to outside developers through B.
I don't think this can be done without using friend. There is no way I know of to make members of a superclass available only to direct inheritors.