Puzzle #2 (based on N4296)

#include <iostream>

struct B
{
  virtual void foo()
  {
    std::cout << "B";
  }
};

struct A : B
{
  virtual void foo() = 0;
};

void A::foo()
{
  std::cout << "A";
}

struct D : A
{
  void foo() override
  {
    std::cout << "D";
  }
};

int main()
{
  B b;
  D d;
  b.foo();
  d.foo();
}

With given code, pick one answer:

Guaranteed to print “AA”
Guaranteed to print “AB”
Guaranteed to print “AD”
Guaranteed to print “BA”
Guaranteed to print “BB”
Guaranteed to print “BD”
Guaranteed to print “DA”
Guaranteed to print “DB”
Guaranteed to print “DD”
Undefined behavior
Implementation defined
Will not compile

Click here for the answer

The correct answer is: Guaranteed to print “BD”.

There are two things that may be (or may not) strange to you:

A pure virtual method definition.
A pure virtual method overriding.

Both of them are well defined in the standard.

A pure virtual method can be defined but it must be defined outside of the class body

10.4.2 (Note)

A function declaration cannot provide both a pure-specifier and a definition

And about the overriding:

10.4.5

(…) a pure virtual function may override a virtual function which is not pure.

C++ puzzle #2

Puzzle #2 (based on N4296)

Thanks for reading o/