container of pointers and virtual member function templates

[Sat]

Hi,

I have a simplified version of a problem that I am facing (hope I haven't
oversimplified it). This code doesn't work now and I want to find how I can
make it work. Can I call the derived class version from the base class
pointer and still be able to use a vector with derived element pointers?

class BaseElem { };

class DerivedElem1 { };

class DerivedElem2 { };

class BaseContainer
{
public:
virtual void GetAllElems(std::vector<BaseElem*>& allElems) const;
};

class DerivedContainer1
{
public:
virtual void GetAllElems(std::vector<DerivedElem1*>& allElems) const;
};

class DerivedContainer2
{
public:
virtual void GetAllElems(std::vector<DerivedElem2*>& allElems) const;
};

 

[David White]

Hi,

I have a simplified version of a problem that I am facing (hope I haven't
oversimplified it). This code doesn't work now and I want to find how I can
make it work. Can I call the derived class version from the base class
pointer and still be able to use a vector with derived element pointers?

class BaseElem { };

class DerivedElem1 { };

class DerivedElem2 { };

You haven't derived either of the last two classes from the first one.

class BaseContainer
{
public:
virtual void GetAllElems(std::vector<BaseElem*>& allElems) const;
};

class DerivedContainer1
{
public:
virtual void GetAllElems(std::vector<DerivedElem1*>& allElems) const;
};

class DerivedContainer2
{
public:
virtual void GetAllElems(std::vector<DerivedElem2*>& allElems) const;
};

Again, neither of your "derived" containers derives from your base
container. In fact, you have no inheritance anywhere in this code.

Even if you specify the inheritance that you appear to have intended for
both elements and containers, your derived-class virtual functions do not
override the base-class virtual function. A vector<BaseElem*> is a type
unrelated to vector<DerivedElem1*>, so if I've understood your question
correctly the answer is "no".

DW

 

[Sat]

Thanks David, You understood the question correctly. The inheritance part
was missing in my code.

I have re-written it here:
class BaseElem { };

class DerivedElem1 : public BaseElem { };

class DerivedElem2 : public BaseElem { };

class BaseContainer
{
public:
virtual void GetAllElems(std::vector<BaseElem*>& allElems) const;
};

class DerivedContainer1 : public BaseContainer
{
public:
virtual void GetAllElems(std::vector<DerivedElem1*>& allElems) const;
};

class DerivedContainer2 : public BaseContainer
{
public:
virtual void GetAllElems(std::vector<DerivedElem2*>& allElems) const;
};

I understand that the std::vector<DerivedElem*> is unrelated to
std::vector<BaseElem*>. I am trying to see if anyone has a good idea of
implementing this in a different way. The intent is to return a bunch of
DerivedElem pointers in a container using a virtual function.

I tried making GetAllElems a templated member function like this:

template<ElemType>
virtual void GetAllElems(std::vector<ElemType>& allElems) const;

but later found that templated member functions cannot be virtual functions.
That's why I am stuck.

Thanks
Sat

 

[David White]

I understand that the std::vector<DerivedElem*> is unrelated to
std::vector<BaseElem*>. I am trying to see if anyone has a good idea of
implementing this in a different way. The intent is to return a bunch of
DerivedElem pointers in a container using a virtual function.

I tried making GetAllElems a templated member function like this:

template<ElemType>
virtual void GetAllElems(std::vector<ElemType>& allElems) const;

but later found that templated member functions cannot be virtual

functions.

That's right, probably because of implementation difficulties for the
compiler in constructing a v-table that can support templates.

That's why I am stuck.

I can't think of any really nice way to do it. The best I can come up with
is a virtual function that takes a std::vector<BaseElem*> &, and a
non-virtual member function template in the base class that calls the
virtual function and converts the vector it creates into another vector of
the correct type of element using a cast. Or, if you want to ensure type
safety at the expense of execution time, you can dynamic_cast every element
separately.

Another, even nastier, method - but fast - would be to directly cast the
std::vector<DerivedElem*> & passed to the template member in the base class
into a std::vector<BaseElem*> & for passing to the virtual function.

I probably need a few slaps across the face from other regulars here or I
might come up with something even worse

DW

 

[David White]

I can't think of any really nice way to do it. The best I can come up with
is a virtual function that takes a std::vector<BaseElem*> &, and a
non-virtual member function template in the base class that calls the
virtual function and converts the vector it creates into another vector of
the correct type of element using a cast.

To clarify, here I meant casting the address of the first element of
std::vector<BaseElem*>, which is a BaseElem**, into a DerivedElem** for
insertion into the std::vector<DerivedElem*> & passed to the template
function. I didn't mean casting one vector type into another, as it
appeared. (Though I meant exactly that later on.)

DW