Template array of pointers c++?

Question

Hello guys in my c++ program I have four classes (A,B,C,D)

• A is the base class
• B inherits from A
• C inherits from A
• D inherits from B

All of them are template classes template<class Type> and each of them has a print method that prints its private members and the private members of the class that it inherits from.

So B would print B private members and A private members, C would print C private members and A private members, D would print its private members and B,A private members.

In the main function I want to create an Array of pointers for class A that have 3 locations for an object of each class then I want to loop each object print method.

the problem is when I change the classes to a template classes I got an error message says "that my classes don't have constructors" ; however they do have them.

here is my code Please Help (Note I commented where the errors occurs for you):

#include <iostream>
#include <string>
using namespace std;

template <class Type>
class A
{
public:
virtual void print()
{
    cout<<"the base class (A) private (x) is : "<<x<<endl;
}

A(Type X = 0)
{
    x = X;
}
void setX(Type X)
{
    x = X;
}

Type getX() const
{
    return x;
}

private:
Type x;
};


template <class Type>
class B:public A
{
public:
B(Type X = 0,Type Y = 0)
{
    setX(X);
    y = Y;
}
void setY(Type Y)
{
    y = Y;
}

Type getY() const
{
    return y;
}

void print()
{
    A::print();

    cout<<"private (y) in class (B) is : "<<getY()<<endl;
}

private:
Type y;
};

template <class Type>
class C:public A
{
public:
C(Type X = 0,Type Z = 0)
{
    setX(X);
    z = Z;
}
void setZ(Type Z)
{
    z = Z;
}

Type getZ() const
{
    return z;
}

void print()
{
    A::print();

    cout<<"private (z) in class (C) is : "<<getZ()<<endl<<endl;
}

private:
Type z;
};


template <class Type>
class D:public B
{
public:
D(Type X = 0,Type Y = 0,Type W = 0)
{
    setX(X);
    setY(Y);
    w = W;
}
void setW(Type W)
{
    w = W;
}

Type getW() const
{
    return w;
}

void print()
{
    B::print();

    cout<<"private (w) in class (D) is : "<<getW()<<endl;
}

private:
Type w;
};


void main()
{
A<int>* arrayOfPointers[3];

arrayOfPointers[0] = new B(1,100);//error here
arrayOfPointers[1] = new C(2,200);//error here
arrayOfPointers[2] = new D(3,300,3000);//error here

for(int i = 0 ; i<3;i++)
{
    cout<<typeid(*arrayOfPointers[i]).name()<<" Print method : \n"<<endl;
    arrayOfPointers[i]->print();
    cout<<"**********************\n"<<endl;
}

}

Answer 1

You forgot two things:

1) Your inheritance needs to specify template arguments for the class they are inheriting from. For example:

template <class Type>
class B : public A<Type>
{
    ...
}

2) When you are instantiating your classes, you need to provide template arguments, too:

arrayOfPointers[0] = new B<int>(1, 100);
arrayOfPointers[1] = new C<int>(2, 200);
arrayOfPointers[2] = new D<int>(3, 300, 3000);

However, you can also provide template functions to instantiate these classes from the provided arguments, like those make_(...) methods from the std library:

template <class Type>
B<Type>* create_B(const Type& t1, const Type& t2)
{
    return new B<Type>(t1, t2);
}

and use it like this:

 arrayOfPointers[0] = create_B(1, 100);

However, be aware that those methods are creating raw pointers to allocated heap memory, so you are responsible for deleting it (you might use shared_ptrs or whatever to overcome that or just return an object etc but this is not actually part of your question/my answer).

Answer 2

You inherit from A, but you should inherit from a specific instantiation A<T>. Maybe class B:public A<Type> ?