[class.default.ctor]

# 11 Classes [[class]](./#class)

## 11.4 Class members [[class.mem]](class.mem#class.default.ctor)

### 11.4.5 Constructors [[class.ctor]](class.ctor#class.default.ctor)

#### 11.4.5.2 Default constructors [class.default.ctor]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1357)

A [*default constructor*](#def:constructor,default "11.4.5.2 Default constructors [class.default.ctor]") 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)[.](#1.sentence-1)

If there is no user-declared constructor or constructor template for classX,
a non-explicit constructor having no parameters is implicitly declared
as defaulted ([[dcl.fct.def]](dcl.fct.def "9.6 Function definitions"))[.](#1.sentence-2)

An implicitly-declared default constructor is an
inline public member of its class[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1374)

A defaulted default constructor for class X is defined as deleted if

- [(2.1)](#2.1)

any non-static data member with no default member initializer ([[class.mem]](class.mem "11.4 Class members")) is
of reference type,

- [(2.2)](#2.2)

X is a non-union class and
any non-variant non-static data member of const-qualified type
(or possibly multidimensional array thereof)
with no [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]") is not const-default-constructible ([[dcl.init]](dcl.init "9.5 Initializers")),

- [(2.3)](#2.3)

any non-variant potentially constructed subobject, except for a non-static data member
with a [*brace-or-equal-initializer*](dcl.init.general#nt:brace-or-equal-initializer "9.5.1 General [dcl.init.general]"),
has class type M (or possibly multidimensional array thereof)
and overload resolution ([[over.match]](over.match "12.2 Overload resolution"))
as applied to find M's corresponding constructor
does not result in a usable candidate ([[over.match.general]](over.match.general "12.2.1 General")), or

- [(2.4)](#2.4)

any potentially constructed subobject S has
class type M (or possibly multidimensional array thereof),M has
a destructor that is deleted or inaccessible from the defaulted default
constructor, and
either S is non-variant or S has a default member initializer[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1401)

A default constructor for a class X is[*trivial*](#def:constructor,default,trivial "11.4.5.2 Default constructors [class.default.ctor]") if it is not user-provided and if

- [(3.1)](#3.1)

X has no virtual functions ([[class.virtual]](class.virtual "11.7.3 Virtual functions")) and no virtual base
classes ([[class.mi]](class.mi "11.7.2 Multiple base classes")), and

- [(3.2)](#3.2)

no non-static data member of X has
a default member initializer ([[class.mem]](class.mem "11.4 Class members")), and

- [(3.3)](#3.3)

all the direct base classes of X have trivial default constructors, and

- [(3.4)](#3.4)

either X is a union or
for all the non-variant non-static data members of X that are of class
type (or array thereof), each such class has a trivial default constructor[.](#3.sentence-1)

Otherwise, the default constructor is[*non-trivial*](#def:constructor,default,non-trivial "11.4.5.2 Default constructors [class.default.ctor]")[.](#3.sentence-2)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1425)

If a default constructor of a union-like class X is trivial,
then for each union U that is either X or an anonymous union member of X,
if the first variant member, if any, of U has implicit-lifetime type ([[basic.types.general]](basic.types.general "6.9.1 General")),
the default constructor of X begins the lifetime of that member
if it is not the active member of its union[.](#4.sentence-1)

[*Note [1](#note-1)*:

It is already the active member if U was value-initialized[.](#4.sentence-2)

— *end note*]

Otherwise,
an implicitly-defined ([[dcl.fct.def.default]](dcl.fct.def.default "9.6.2 Explicitly-defaulted functions")) 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*](class.base.init#nt:ctor-initializer "11.9.3 Initializing bases and members [class.base.init]") ([[class.base.init]](class.base.init "11.9.3 Initializing bases and members")) and an empty[*compound-statement*](stmt.block#nt:compound-statement "8.4 Compound statement or block [stmt.block]")[.](#4.sentence-3)

If that user-written default constructor would be ill-formed,
the program is ill-formed[.](#4.sentence-4)

If that user-written default constructor would be constexpr-suitable ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers")),
the implicitly-defined
default constructor is constexpr[.](#4.sentence-5)

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 are implicitly defined[.](#4.sentence-6)

[*Note [2](#note-2)*:

An implicitly-declared default constructor has an
exception specification ([[except.spec]](except.spec "14.5 Exception specifications"))[.](#4.sentence-7)

An explicitly-defaulted definition might have an
implicit exception specification, see [[dcl.fct.def]](dcl.fct.def "9.6 Function definitions")[.](#4.sentence-8)

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1458)

[*Note [3](#note-3)*:

A default constructor is implicitly invoked to initialize
a class object when no initializer is specified ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#5.sentence-1)

Such a default constructor needs to be accessible ([[class.access]](class.access "11.8 Member access control"))[.](#5.sentence-2)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L1466)

[*Note [4](#note-4)*:

[[class.base.init]](class.base.init "11.9.3 Initializing bases and members") 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[.](#6.sentence-1)

— *end note*]