The following code example is taken from the book
C++17 - The Complete Guide
by Nicolai M. Josuttis,
Leanpub, 2017
The code is licensed under a
Creative Commons Attribution 4.0 International License.
// raw code
struct Derived;
struct Base {
friend struct Derived;
private:
Base() {
}
};
struct Derived : Base {
};
int main()
{
Derived d1{}; // ERROR since C++17
Derived d2; // still OK (but might not initialize)
}