defaulted default constructor ? in n3290 draft

Question

A point from n3290 draft §12.1 (Constructors) ¶5:

An implicitly-declared default constructor is an inline public member of its class. A defaulted default constructor for class X is defined as deleted if:

• X is a union-like class that has a variant member with a non-trivial default constructor,
• any non-static data member with no brace-or-equal-initializer is of reference type,
• any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or-equal-initializer does not have a user-provided default constructor,
• X is a union and all of its variant members are of const-qualified type (or array thereof),
• X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),
• any direct or virtual base class, or non-static data member with no brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution (13.3) as applied to M’s default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
• any direct or virtual base class or non-static data member has a type with a
  destructor that is deleted or inaccessible from the defaulted default constructor

Please explain the defaulted default constructor with some example program.

Answer 1

I think this excerpt from Wikipedia explains this:

Explicitly-defaulted and deleted special member functions

In C++03, the compiler provides, for classes that do not provide for themselves, a default constructor, a copy constructor, a copy assignment operator (operator=), and a destructor. The programmer can override these defaults by defining custom versions. C++ also defines several global operators (such as operator= and operator new) that work on all classes, which the programmer can override.

However, there is very little control over the creation of these defaults. Making a class inherently non-copyable, for example, requires declaring a private copy constructor and copy assignment operator and not defining them. Attempting to use these functions is a violation of the one definition rule. While a diagnostic message is not required,[5] this typically results in a linker error.[citation needed]

In the case of the default constructor, the compiler will not generate a default constructor if a class is defined with any constructors. This is useful in many cases, but it is also useful to be able to have both specialized constructors and the compiler-generated default.

C++11 will allow the explicit defaulting and deleting of these special member functions. For example, the following type explicitly declares that it is using the default constructor:

Code Example:

struct SomeType 
{
    SomeType() = default; //The default constructor is explicitly stated.
    SomeType(OtherType value);
};

---

Since you seem to be a Standerdese fan(Almost all of your Questions seek explanations on Standard Quotes) this paper here about how standards committe arrive to defining default and deleted functions should be a good read for you:

Defaulted and Deleted Functions