defaulted default constructor ? in n3290 draft

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-vari开发者_Go百科ant 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.

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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);
};

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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

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One of the special member functions is "defaulted" if it is declared with the = default; syntax. The line right before the first line you quoted states:

If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4).

Therefore, a "defaulted" default constructor is a default constructor (constructor that can be called with no arguments) that is declared with = default. This can be explicitly defined using the = default syntax, or implicitly defined, per the above line.