Accessing protected data member of an abstract class template fail to compile

Question

I was playing with templates and decided to make an abstract class template(AbstractOperation) with protected data members. Now I inherit them publically to Addition but when I try to access the protected data members of base class directly inside a member function of derived class it fails to compile ie return mLhs + mRhs;, but return this->mLhs +this->mRhs; works fine in int Addition<oprand>::operation().

So I repeated the same class hierarchy without templates now I can access the members just fine.class A and class B , int fun() { return m; }

So what I'm trying to understand is why compilation fails for the line return mLhs + mRhs;

Below is my sample code , online link

#include <iostream>

// template implementation
template <typename oprand>
class AbstractOperation
{
protected:
    oprand mLhs;
    oprand mRhs;

public:
    virtual int operation() = 0;
    AbstractOperation(oprand x, oprand y) : mLhs(x), mRhs(y) {}
    oprand getLhs() const { return mLhs; }
    oprand getRhs() const { return mRhs; }
};

template <typename oprand>
class Addition : public AbstractOperation<oprand>
{
public:
    Addition(oprand x, oprand y) : AbstractOperation<oprand>(x, y) {}
    int operation();
};

template <typename oprand>
int Addition<oprand>::operation()
{
    return mLhs + mRhs; // fails to compile
    // return this->mLhs +this->mRhs;     // compile successfully
    // return AbstractOperation<oprand>::mLhs + AbstractOperation<oprand>::mRhs;  //compile successfully
}

// normal implementation
class A
{
protected:
    int m;

public:
    A(int val) : m(val) {}
    virtual int fun() = 0;
};

class B : public A
{
public:
    B(int val) : A(val) {}
    int fun() { return m; }
};

int main()
{
    A *a = new B(5);
    std::cout << a->fun() << "\n";
    AbstractOperation<int> *p = new Addition<int>(4, 5);
    std::cout << p->operation() << "\n";
    delete a;
    delete p;
}

Error

main.cpp: In member function ‘int Addition<oprand>::operation()’:
main.cpp:28:12: error: ‘mLhs’ was not declared in this scope
     return mLhs + mRhs; // fails to compile
            ^~~~
main.cpp:28:19: error: ‘mRhs’ was not declared in this scope
     return mLhs + mRhs; // fails to compile
                   ^~~~

Accessing protected data member of an abstract class template fail to compile

0 Answers0