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-specifieropt attribute-specifier-seqopt
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:
in a member-declaration that belongs to the member-specification of a class but is not a friend declaration, the id-expression is the injected-class-name of the immediately-enclosing class;
in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation of the immediately-enclosing class template; or
in a declaration at namespace scope or in a friend declaration, the id-expression is a qualified-id that names a constructor ([class.qual]).
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:
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 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 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 and no virtual base classes, and
no non-static data member of its class has a default member initializer, 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 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 ]