Question

Suppose I have a class template which have a member pData, which is an AxB array of arbitary type T.

template <class T> class X{ 
public:
    int A;
    int B;
    T** pData;
    X(int a,int b);
    ~X();        
    void print(); //function which prints pData to screen

};  
template<class T>X<T>::X(int a, int b){ //constructor
    A = a;
    B = b;
    pData = new T*[A];
    for(int i=0;i<A;i++)
        pData[i]= new T[B];
    //Fill pData with something of type T
}
int main(){
    //...
    std::cout<<"Give the primitive type of the array"<<std::endl;
    std::cin>>type;
    if(type=="int"){
        X<int> XArray(a,b);
    } else if(type=="char"){
        X<char> Xarray(a,b);
    } else {
        std::cout<<"Not a valid primitive type!";
    } // can be many more if statements.
    Xarray.print() //this doesn't work, as Xarray is out of scope.
}

As the instance Xarray is constructed inside of an if statement, I cannot use it anywhere else. I tried to make a pointer before the if statements but as the type of the pointer is unknown at that point, I did not succeed.

What would be a proper way of dealing with this kind of a problem?

Answer 1

C++ is a statically typed language, meaning that you must know the type of objects at compile time. In this case you are basing the type of the object constructed on user input, so it’s not possible to know the type at runtime.

The most common way to address this issue is to use dynamic polymorphism in which functions are invoked via a common interface using late binding. We accomplish this in C++ using virtual functions. For example:

struct IPrintable {
   virtual void print() = 0;
};

template<class T>
class X : public IPrintable {
  // Same code as you showed above.
};

int main() {
  std::cout<<"Give the primitive type of the array"<<std::endl;
  std::cin>>type;

  std::unique_ptr<IPrintable> XArray;

  if(type=="int"){
      XArray.reset(new X<int>(a,b));
  } else if(type=="char"){
      XArray.reset(new X<char>(a,b));
  } else {
      std::cout<<"Not a valid primitive type!";
  } // can be many more if statements.

  Xarray->print() // this works now!
}

This solves the out of scope issue and allows you to print using the dynamic type of the XArray variable. Virtual functions are the secret sauce that make this possible.