Question

let’s say i want to have a member variable for a pointer to std::vector but i do not want to specify what type of variable it stores. I want to access only those functions that are independant of it’s actual generic type. is this possible with c++? something like this:

class Foo{
public:
    void setVec(std::vector* someVec){
        myVec = someVec;
    };
    int getSize(){
        return myVec.size();
    };
private:
    std::vector* myVec;
};


int main(){
    Foo foo;
    vector<int> vec1;
    vector<float> vec2;
    foo.setVec(&vec1);
    cout<<foo.getSize();
    foo.setVec(&vec2);
    cout<<foo.getSize();
}

note: i do not want to template Foo and i want to use only a single instance of Foo with vectors of different type.

of course – if I could alter the class vector then i could create an untemplated baseclass

class Ivector{
    virtual int size()=0;
};

and then make the

class vector<T> : public IVector...

inherit from Ivector. but what do I do if i can’t alter the class in question and the templated class does not have such an untemplated baseclass?

thanks!

Answer 1

You are almost at the answer. Instead of making std::vector inherit from Ivector, create a new class:

template <typename T>
class IVectorImpl : public Ivector
{
public:
    explicit IVectorImpl(std::vector<T> * Data) : m_Data(Data){}
    std::vector<T> * m_Data;
 ...
     virtual int size() const {return m_Data->size();}
  // Implement all the Ivector functions here to call the respective functions off of m_Data
};

Now have your Foo class keep a pointer to Ivector instead of std::vector.

Make Foo::setVec templated

template <typename T>
void setVec(std::vector<T> * vec)
{
   Ivector * newVec = new IVectorImpl<T>(vec);
   delete myVec;
   myVec = newVec;
}