C++ Override Template class virtual function in Template class

Question

I am trying to override functions in my program in the method below but when compiling it shows variable1 as undefined in the Derived clear function.

I have been able to get this to work when changing the derived class to a non-template, but then I am missing the DATATYPE and VALUE variables when initiating the template for Base.

template <class DATATYPE=short, short VALUE = 100>
class Base {
    public:
    Base();

    virtual void clear()=0;
    
    protected:
    int variable1;
}


template <class DATATYPE=short, short VALUE = 100>
class Derived : public Base<DATATYPE, VALUE> {
    public:
    Derived();

    void clear() {
        int testVar = variable1;
    }
}

Anyway for your specific error, take a look at [this question](https://stackoverflow.com/questions/4643074/why-do-i-have-to-access-template-base-class-members-through-the-this-pointer) for why accessing members of base classes that are template instantiations requires a `this->` prefix. — Nathan Pierson, Sep 13 '21 at 17:12

Pepijn Kramer · Accepted Answer · 2021-09-13T17:59:46.247

A compiling version of your code:

template <typename data_t, short value_v>
class Base 
{
public:
    Base() = default;               // <== was missing impl, set to default
    virtual ~Base() = default;      // classes with virtual methods must have virtual destructor
    virtual void clear() = 0;

protected:
    int variable1;
}; 


template <typename data_t = short, short value_v = 100>
class Derived : 
    public Base<data_t, value_v>
{
    // by adding a using you can more easily change the template params
    // without having to update all your code. Helps keeping code more readable too
    using base_t = Base<data_t, value_v>;

public:
    void clear() override // <== missing override
    {
        //  different ways of accessing member in template base class
        int v1 = Base<data_t, value_v>::variable1;
        int v2 = this->variable1;
        int v3 = base_t::variable1;
    }
}; 


int main()
{
    Derived<short,100> d;
    d.clear();
}

The `virtual` keyword is not required when `override` is present. — Kyle, Sep 13 '21 at 17:40

C++ Override Template class virtual function in Template class

1 Answers1