How can I get derived class object from my generic interface pointer.
I do not want to type cast the generic interface in order to achieve above .
e.g:
class Base {}; // Has pure virtual functions and is my interface class
class Derived : public Base {}; // Additional functions .
Base *b = new Derived();
Now, I want to get derived pointer Derived *derived from b without casting.
How can I achieve this?
Your setup seems strange, I'd look for alternative options like returning Derived directly or moving the functions you need into base since you seem to need them.
If you still want to do that you could have something like:
class Derived; // Forward declaration
class Base {
public:
virtual Derived* as_derived() { return null; }
...
};
class Derived : public Base {
public:
Derived* as_derived() override { return this; }
};
Still, this looks like a hack and I would suggest casting instead. At least casting it's a common pattern and it will be easier for others to understand.
Related
I have been struggling with this kind of problem for a long time, so I decided to ask here.
class Base {
virtual ~Base();
};
class Derived1 : public Base { ... };
class Derived2 : public Base { ... };
...
// Copies the instance of derived class pointed by the *base pointer
Base* CreateCopy(Base* base);
The method should return a dynamically created copy, or at least store the object on stack in some data structure to avoid "returning address of a temporary" problem.
The naive approach to implement the above method would be using multiple typeids or dynamic_casts in a series of if-statements to check for each possible derived type and then use the new operator.
Is there any other, better approach?
P.S.: I know, that the this problem can be avoided using smart pointers, but I am interested in the minimalistic approach, without a bunch of libraries.
You add a virtual Base* clone() const = 0; in your base class and implement it appropriately in your Derived classes. If your Base is not abstract, you can of course call its copy-constructor, but that's a bit dangerous: If you forget to implement it in a derived class, you'll get (probably unwanted) slicing.
If you don't want to duplicate that code, you can use the CRTP idiom to implement the function via a template:
template <class Derived>
class DerivationHelper : public Base
{
public:
virtual Base* clone() const
{
return new Derived(static_cast<const Derived&>(*this)); // call the copy ctor.
}
};
class Derived1 : public DerivationHelper <Derived1> { ... };
class Derived2 : public DerivationHelper <Derived2> { ... };
An alternative is to have a pure virtual CreateCopy() method in the common base that is implemented in each derived class.
Is it an O.K. design if i return an instance of Derived class from Base class member function(created using new Derived()) ?
Also this involves the need to forward declare the Derived class and also make the Derived class constructor public.
Code Sample :-
Class Base
{
public:
Base() {}
Derived* get_new_derived(int X)
{
Derived *der = new Derived(X);
return der;
}
}
Class Derived : public Base
{
private:
int _X;
protected:
Derived(int X) { _X = X; }
public:
Derived* get_new_derived(int X)
{
Derived *der = new Derived(X);
return der;
}
}
I have one more derived class(say Derived1). Now let's say :-
Usage1 :
Derived1* der1 = new Derived1();
Derived * der = der1->get_new_derived(10);
Usage2 :
Derived1* der1 = new Derived1();
Base* bs = der1;
Derived * der = bs->get_new_derived(10);
In the general case, this usually indicates bad design - a base class shouldn't normally know/care about its derived classes.
Note that you can return a pointer to Derived typed as Base* - that's the principle of the Factory Method design pattern. Perhaps this fits your actual scenario?
Note however that all rules have exceptions. If your (base class + derived classes) represent a tighly coupled and coherent unit (such as a Tree interface with two subclasses FastTree and SmallTree, optimised for different purposes), it is acceptable for them to be aware of each other, and Tree defining such functions as:
std::unique_ptr<SmallTree> Tree::createSmallCopy() const
{
return { new SmallTree(/*args*/) };
}
std::unique_ptr<FastTree> Tree::createFastCopy() const
{
return { new FastTree(/*args*/) };
}
So it depends on your actual use case. I'd generally be wary of such design, it but it has its uses.
As a side note, in modern C++, you should never use an owning raw pointer - use a smart pointer like std::unique_ptr or boost::shared_ptr.
How about this?
// The Curiously Recurring Template Pattern (CRTP)
template<class Derived>
class Base
{
// methods within Base can use template to access members of Derived
};
class Derived : public Base<Derived>
{
// ...
};
http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
Yes you can do it. The question is whether you should.
You shouldn't do it since it implies Base knows Derived which isn't proper object oriented design. The dependencies should be one way - derived knows the Base and not vice versa.
How do you design polymorphism when you have a member which type depends on some constraints.
Say I have this :
template<typename T>
class Base
{
public:
Base() = default;
virtual ~Base() = default;
T member;
};
class DerivedA : public Base<int>
{
public:
DerivedA() {member = 5;}
};
class DerivedB : public Base<float>
{
public:
DerivedB() = default;
};
I want to be able to create a new derived object depending on different parameters, i.e :
Base *b;
if (something)
b = new DerivedA();
else
b = new DerivedB();
Obviously I can't do this since I need to provide template parameters for the declaration of b.
Is this bad design ? How do you handle this ?
I could write a small wrapper :
class Wrapper() {};
template<typename T>
class Base : public Wrapper
{
// ...
};
Wrapper a, b;
a = new DerivedA;
b = new DerivedB;
But then I won't have access directly to member or other methods declared in Base or Derived. I would need to cast : reinterpret_cast<DerivedA*>(a)->member, making polymorphism useless.
Thanks
Wrapper design should be exactly what you are looking for. The problem is that c++ is statically typed, so you can't declare member without specifying its type. The common way to avoid this is to design a base class which supports all the functionality you want, and implement a specific behaviour in derived classes, just as you did.
Probably the problem is that your class doesn't support all the functionality you need. Try to avoid using member directly and wrap its usage into virtual methods. Reimplement these methods in your derived classes.
If this is still not an option, consider methods for extracting your member. Maybe virtual getters and setters with appropriate conversions.
As a last resort, consider boost::variant, boost::any. But really they are implemented using the technique similar to your wrapper. So you get wrapper for the wrapper.
Well, if the "access" depends on the template parameter T (such reading Base.member), then you have to supply it somehow. Casting to one of the derived classes is one way to do it, but you don't need the reinterpret_cast. You need to start using pointers/references, to avoid the cut-off and let the substitutability work correctly:
Wrapper *a, *b;
a = new DerivedA;
b = new DerivedB;
int a_member = static_cast<DerivedA*>(a)->member;
float b_member = static_cast<DerivedB*>(b)->member;
And if you add a virtual method to Wrapper to make it polymorphic, you can do dynamic cast as well:
DerivedB* b_derived = dynamic_cast<DerivedB*>(b);
if (b_derived != nullptr) {
float b_member = b_derived->member;
// ...
}
I have been struggling with this kind of problem for a long time, so I decided to ask here.
class Base {
virtual ~Base();
};
class Derived1 : public Base { ... };
class Derived2 : public Base { ... };
...
// Copies the instance of derived class pointed by the *base pointer
Base* CreateCopy(Base* base);
The method should return a dynamically created copy, or at least store the object on stack in some data structure to avoid "returning address of a temporary" problem.
The naive approach to implement the above method would be using multiple typeids or dynamic_casts in a series of if-statements to check for each possible derived type and then use the new operator.
Is there any other, better approach?
P.S.: I know, that the this problem can be avoided using smart pointers, but I am interested in the minimalistic approach, without a bunch of libraries.
You add a virtual Base* clone() const = 0; in your base class and implement it appropriately in your Derived classes. If your Base is not abstract, you can of course call its copy-constructor, but that's a bit dangerous: If you forget to implement it in a derived class, you'll get (probably unwanted) slicing.
If you don't want to duplicate that code, you can use the CRTP idiom to implement the function via a template:
template <class Derived>
class DerivationHelper : public Base
{
public:
virtual Base* clone() const
{
return new Derived(static_cast<const Derived&>(*this)); // call the copy ctor.
}
};
class Derived1 : public DerivationHelper <Derived1> { ... };
class Derived2 : public DerivationHelper <Derived2> { ... };
An alternative is to have a pure virtual CreateCopy() method in the common base that is implemented in each derived class.
This compiles fine, although I wouldn't want to try running it just yet. However ...
//class base;
//class derived;
//class derived : public base;
class base {};
class derived : public base {};
class other
{
public:
void func() {base1 = derived1;}
base* base1;
derived* derived1;
};
void main()
{
}
... moving the class other to above the definition of base and derived for which there is a similar thing I must do in a program of myne causes compile errors.
The obvious solution is to forward declare base and derived shown commented out at the top of the code, however this causes a can't convert between base* and derived* error. Attempting to forward declare including the inheritance information dosn't work either.
This should work. You need to move other up
BUT declare func below. That way func is able to "see" that derived is of type base.
e.g.,
class base;
class derived;
//class derived : public base;
class other
{
public:
void func();
base* base1;
derived* derived1;
};
class base {};
class derived : public base {};
void other::func() { base1 = derived1; }
There is no syntax for forward-declaring two classes and specifying that one inherits from the other. This is because inheritance can be complex (multiple and/or virtual), so the assignment base1 = derived1 might involve some arithmetic, and it's difficult/impossible for the compiler to generate that arithmetic when it only knows that derived inherits from base.
So, to solve your problem, you can make func not inline (see the answer from AbstractDissonance), or, if you absolutely need it to be inline, use reinterpret_cast:
class base;
class derived;
class other
{
public:
void func() {base1 = reinterpret_cast<base*>(derived1);}
base* base1;
derived* derived1;
};
This is bad because it is not guaranteed to work by the C++ standard, and it will almost certainly not work if you use multiple/virtual inheritance. Use this only if func has to be as fast as possible (i.e. inline), and if you don't need your code to be portable.