Protected member is "not declared in this scope" in derived class

Question

#include <vector>
#include <iostream>

template <class T>
class Base
{
protected:
    std::vector<T> data_;
};

template <class T>
class Derived : public Base<T>
{
public:
    void clear()
    {
        data_.clear();
    }
};

int main(int argc, char *argv[])
{
    Derived<int> derived;
    derived.clear();
    return 0;
}

I cannot compile this program. I get:

main.cpp:22: error: 'data_' was not declared in this scope

Please, could you explain why data_ is not visible in the Derived class?

Solution 1

To fix this, you need to specify Base<T>::data_.clear() or this->data_.clear(). As for why this happens, see here.

Solution 2

In the case of templates compiler is not able to determine if the member is really coming from base class. So use this pointer and it should work:

void clear()
{
   this->data_.clear();
}

When compiler looks the Derived class definition, it doesn't know which Base<T> is being inherited (as T is unknown). Also data_ is not any of the template parameter or globally visible variable. Thus compiler complains about it.