Need an example showing that default constructor is not inherited

Question

I know that default constructor is not inherited, as stated in n3337.

And there is an example there:

struct B2 {
  B2(int = 13, int = 42);
};

struct D2 : B2 {
  using B2::B2;
};

With quite good explanation:

The candidate set of inherited constructors in D2 for B2 is

...
—B2(int = 13, int = 42)
—B2(int = 13)
—B2()

And most important:

The set of constructors present in D2 is
—D2(), implicitly-declared default constructor, not inherited

For me this example does not show the difference, in a sense that even if this very constructor was inherited - its behavior was not different from the implicitly-declared default constructor.

I need an example showing the difference in the way that can be easily understand for, let say, an audience familiar with C++03 but wanting to learn C++11.

---

[UPDATE]
All answers (including my own) are of kind "if default c-tor was inherited then the example would compile/not compile".

I'd prefer answers where the outcome (observable behavior) is different than it would be otherwise.

Answer 1

Consider:

struct foo
{
    foo() {}
    foo(int) {}
};

struct bar : foo
{
    using foo::foo;
};

int main()
{
    bar b;
}

This compiles: Since bar has no user-declared constructors, a default constructor will be declared implicitly.

struct foo
{
    foo() {}
    foo(int) {}
};

struct bar : foo
{
    using foo::foo;
    bar(double) {}
};

int main()
{
    bar b;
}

This does not compile. The default constructor is not inherited, and it is not declared implicitly, since there is the bar(double) constructor.

Answer 2

One possible difference: multiple-constructor-inheritance from classes with default constructors. For example:

struct A { A(int=0); };
struct B { B(double=3.14); };
struct C : A, B {
  using A::A;
  using B::B;
};

C c;

If default constructors were inherited, C would inherit one from both A and B, resulting in ambiguity.

I can't think of a use case for multiple-constructor-inheritance, so this may not be the perfect example you're looking for, but it's something.

Answer 3

Here is the example, that can be produced from the following feature of inherited constructors:

12.9 Inheriting constructors
[...]
4) A constructor so declared has the same access as the corresponding constructor in X.

So my proposal is to have protected default constructor in base:

class Base {
protected:
    Base(int) {}
    Base() = default;
};

If this constructor was derived, then derived class cannot be instantiated because derived constructor would have protected access. If not derived - then default implicitly declared constructor has public access:

struct Derived : Base {
    using Base::Base;
};

int main() {
    Derived d1{};  // not inherited, default constructor is public
    Derived d2{1}; // not compiling since this c-tor is inherited, thus protected
}