15 Special member functions [special]

15.1 Constructors [class.ctor]

Constructors do not have names. In a declaration of a constructor, the declarator is a function declarator of the form

ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq

where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:

The class-name shall not be a typedef-name. In a constructor declaration, each decl-specifier in the optional decl-specifier-seq shall be friend, inline, explicit, or constexpr. [Example:

struct S {
  S();              // declares the constructor
};

S::S() { }          // defines the constructor

end example]

A constructor is used to initialize objects of its class type. Because constructors do not have names, they are never found during name lookup; however an explicit type conversion using the functional notation will cause a constructor to be called to initialize an object. [Note: For initialization of objects of class type see [class.init]. end note]

A constructor can be invoked for a const, volatile or const volatile object. const and volatile semantics ([dcl.type.cv]) are not applied on an object under construction. They come into effect when the constructor for the most derived object ends.

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:

A default constructor is trivial if it is not user-provided and if:

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 to create an object of its class type ([intro.object]) 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, 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. [Note: 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]. end note]

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, are called to create class objects of dynamic storage duration created by a new-expression in which the new-initializer is omitted ([expr.new]), or are called when the explicit type conversion syntax is used. A program is ill-formed if the default constructor for an object is implicitly used and the constructor is not accessible.

[Note: [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. end note]

A return statement in the body of a constructor shall not specify a return value. The address of a constructor shall not be taken.

A functional notation type conversion can be used to create new objects of its type. [Note: The syntax looks like an explicit call of the constructor. end note] [Example:

complex zz = complex(1,2.3);
cprint( complex(7.8,1.2) );

end example]

An object created in this way is unnamed. [Note: [class.temporary] describes the lifetime of temporary objects. end note] [Note: Explicit constructor calls do not yield lvalues, see [basic.lval]. end note]

[Note: Some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor]. end note]

During the construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified. [Example:

struct C;
void no_opt(C*);

struct C {
  int c;
  C() : c(0) { no_opt(this); }
};

const C cobj;

void no_opt(C* cptr) {
  int i = cobj.c * 100;         // value of cobj.c is unspecified
  cptr->c = 1;
  cout << cobj.c * 100          // value of cobj.c is unspecified
       << '\n';
}

extern struct D d;
struct D {
  D(int a) : a(a), b(d.a) {}
  int a, b;
};
D d = D(1);                     // value of d.b is unspecified

end example]