Template classes vs Private inheritance
#include <iostream>

using namespace std;

template <class T>
class TestImpl {
public: // It wont make a difference even if we use a protected access specifier here
    size_t vval_;
    TestImpl(size_t val = 0) : vval_(val) { }
};

template <class T>
class Test : public TestImpl<T> {
public:
    Test(size_t val) : TestImpl<T>(val) {
        cout << "vval_ : " << vval_ << endl; // Error: vval_ was not declared in this scope
        //! cout << "vval_ : " << TestImpl<T>::vval_ << endl; // this works, obviously
    }
};

int main() {
    Test<int> test1(7);

    return 0;
}

#include <iostream>

using namespace std;

class TestImpl {
public: // It wont make a difference even if we use a protected access specifier here
    TestImpl(size_t val = 0) : vval_(val) {}
    size_t vval_;
};

class Test : public TestImpl {
public:
    Test(size_t val) : TestImpl(val) {
        cout << "vval_ : " << vval_ << endl;
    }
};

int main() {
    Test test1(7);

    return 0;
}