10 Classes [class]

10.3 Class members [class.mem]

10.3.4 Constructors [class.ctor]

10.3.4.1 Default constructors [class.default.ctor]

A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters).
If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).
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 that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
  • X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
  • any non-static data member with no default member initializer ([class.mem]) 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 potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution ([over.match]) as applied to find M's corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
  • any potentially constructed subobject has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.
A default constructor is trivial if it is not user-provided and if:
  • its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • no non-static data member of its class has a default member initializer ([class.mem]), and
  • all the direct base classes of its class have trivial default constructors, and
  • for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
Otherwise, the default constructor is non-trivial.
A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.
The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer and an empty compound-statement.
If that user-written default constructor would be ill-formed, the program is ill-formed.
If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr.
Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined.
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An implicitly-declared default constructor has an exception specification ([except.spec]).
An explicitly-defaulted definition might have an implicit exception specification, see [dcl.fct.def].
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Default constructors are called implicitly to create class objects of static, thread, or automatic storage duration ([basic.stc.static], [basic.stc.thread], [basic.stc.auto]) defined without an initializer ([dcl.init]), are called to create class objects of dynamic storage duration ([basic.stc.dynamic]) created by a new-expression in which the new-initializer is omitted ([expr.new]), or are called when the explicit type conversion syntax ([expr.type.conv]) is used.
A program is ill-formed if the default constructor for an object is implicitly used and the constructor is not accessible ([class.access]).
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[class.base.init] describes the order in which constructors for base classes and non-static data members are called and describes how arguments can be specified for the calls to these constructors.
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