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class B 
private:
 friend class C;
 B() = default;
;

class C : public B ;
class D : public B ;

int main() 
 C ;
 D ;
 return 0;

I assumed that since only class C is a friend of B, and B's constructor is private, then only class C is valid and D is not allowed to instantiate B. But that's not how it works. Where am I wrong with my reasoning, and how to achieve this kind of control over which classes are allowed to subclass a certain base?

Update: as pointed out by others in the comments, the snippet above works as I initially expected under C++14, but not C++17. Changing the instantiation to C c; D d; in main() does work as expected in C++17 mode as well.

This is a new feature added to C++17. What is going on is C is now considered an aggregate. Since it is an aggregate, it doesn't need a constructor. If we look at [dcl.init.aggr]/1 we get that an aggregate is

An aggregate is an array or a class with

• no user-provided, explicit, or inherited constructors ([class.ctor]),




• no private or protected non-static data members (Clause [class.access]),




• no virtual functions, and




• no virtual, private, or protected base classes ([class.mi]).

[ Note: Aggregate initialization does not allow accessing protected and private base class' members or constructors.  — end note ]

And we check of all those bullet points. You don't have any constructors declared in C or D so there is bullet 1. You don't have any data members so the second bullet doesn't matter, and your base class is public so the third bullet is satisfied.

The change that happened between C++11/14 and C++17 that allows this is that aggregates can now have base classes. You can see the old wording here where it expressly stated that bases classes are not allowed.

We can confirm this by checking the trait std::is_aggregate_v like

int main()

 std::cout << std::is_aggregate_v<C>;

which will print 1.

Do note that since C is a friend of B you can use

C c;
C c1;
C c2 = C();

As valid ways to initialize a C. Since D is not a friend of B the only one that works is D d; as that is aggregate initialization. All of the other forms try to default initialize and that can't be done since D has a deleted default constructor.

From What is the default access of constructor in c++:

If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted. An implicitly-declared default constructor is an inline public member of its class.

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. [...] An implicitly-declared copy/move constructor is an inline public member of its class.

Constructors for classes C and D are generated internally by compiler.

BTW.: If you want to play with inheritance, please make sure you have virtual destructor defined.