Inheriting Constructors (revision 1)

Problem Description

There are also a couple of cases where the forwarding constructors cannot be declared by the user, such as for a dependant base class where a class template derives from one (or more) of its own template type parameters.

Outline of the Solution

The basic idea is to extend using declarations to support constructors. This falls out of the grammar today (no grammar changes required) and simply extends the semantic rules to cover the new case.

When a using declaration names a base class constructor, a number of forwarding constructors are implicitly declared in the derived class. Like other implicitly declared constructors, they are only implicitly defined if they are used.

Copy and default constructors are not forwarded, deferring to the existing rules for implicitly declaring copy/default constructors.

As forwarding constructors are implicitly declared, they do not inhibit the implicit declaration of the default constructor.

Forwarding constructors retain the throw spec and explicitness of the base constructor.

User declared constructors inhibit forwarding for that particular signature, much as user declaration of a function would hide a specific signature when using declarations are used with regular functions.

It is an error for multiple using declarations to refer to the same base class. If multiple using directives declare the same signature, the program is ill-formed, even if those constructors are not used. This can be worked around with a user-declared constructor instead.

A program is ill-formed if a fowarding constructor is used that forwards to a base class constructor that is declared private. Note that this is different to the using declarations with regular functions, that are ill-formed even if the private function is not used. It is consistent with the way a variadic constructor template would work though.

Typically, forwarding constructor definitions for classes with virtual bases will be ill-formed, unless the virtual base supports default initialization, or the virtual base is a direct base, and named as the base forwarded-to. Likewise, all data members and other direct bases must support default initialization, or any attempt to use a forwarding constructor will be ill-formed. Note: ill-formed when used, not declared.

Note that typedef names that alias a direct base will work in the same way, and it is even possible to mix and match names either side of the ::.
For example:

struct derived : base {
  typedef base inherited;
};

• using base::base;
• using base::inherited;
• using inherited::base;
• using inherited::inherited;

Proposed Changes to the Working Paper

• handle constexpr
• properly forward rvalues
• handle name lookup
• does not address deleted constructors yet; needs update to 12.9 to copy deletedness to derived class constructor similar to explicit and constexpr

Change 3.4.3.1 class.qual paragraph 2 as indicated:

In a lookup in which the constructor is an acceptable lookup result, if the nested-name-specifier nominates a class C, and additional conditions apply to the name specified after the nested-name-specifier. If that name, when looked up in C, is the injected-class-name of C (clause 9), the name is instead considered to name the constructor of class C. [ Note: For example, the constructor is not an acceptable lookup result in an elaborated-type-specifier so the constructor would not be used in place of the injected-class-name. ] Furthermore, if the nested-name-specifier appears in a using-declaration and that name, when looked up in C, nominates (possibly cv-qualified) class C, the name is instead considered to name the constructor of class C. If that lookup does not find the name, the name is also looked up in the context of the entire using-declaration. If the name is found and nominates the (possibly cv-qualified) class C, the name is instead considered to name the constructor of class C. Such a constructor name shall be used only in the declarator-id of a declaration that names a constructor or in a using-declaration that names a constructor.

A using-declaration introduces a name into the declarative region in which the using-declaration appears. That name is a synonym for the name of some entity declared elsewhere.

using-declaration:
  using typenameopt ::opt nested-name-specifier unqualified-id ;
  using :: unqualified-id ;

The member name specified in a using-declaration is declared in the declarative region in which the using-declaration appears. [Note: only the specified name is so declared; specifying an enumeration name in a using-declaration does not declare its enumerators in the using-declaration's declarative region. -- end note ] Moreover, if a using-declaration names a constructor (3.4.3.1 class.qual), it implicitly declares a set of constructors in the class in which the using-declaration appears (12.9 class.fwd).

In a using-declaration used as a member-declaration, the nested-name-specifier shall name a base class of the class being defined. If such a using-declaration names a constructor, the nested-name-specifier shall name a direct base class of the class being defined, otherwise it Such a using-declaration introduces the set of declarations found by member name lookup (10.2, 3.4.3.1).

[ Note: since constructors and Since destructors do not have names, a using-declaration cannot refer to a constructor or a destructor for a base class. Since specializations of member templates for conversion functions are not found by name lookup, they are not considered when a using-declaration specifies a conversion function (14.5.2). -- end note ] ...

When a using-declaration brings names from a base class into a derived class scope, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name, parameter-type-list (8.3.5), and cv-qualification in a base class (rather than conflicting). [Note: For using-declarations that name a constructor, see 12.9 class.fwd]

All instances of the name mentioned in a using-declaration shall be accessible. In particular, if a derived class uses a using-declaration to access a member of a base class, the member name shall be accessible. If the name is that of an overloaded member function, then all functions named shall be accessible. The base class members mentioned by a using-declaration shall be visible in the scope of at least one of the direct base classes of the class where the using-declaration is specified. [Note: Since constructors do not have names, these access rules do not apply to constructors, see 12.9 class.fwd] ...

The alias created by the using-declaration has the usual accessibility for a member-declaration. A constructor declared by a using-declaration has the accessibility as if it were declared with the access-specifier that applied to the corresponding constructor in the base class.

12.9 Forwarding Constructors [class.fwd]

A using-declaration (7.3.3 namespace.udecl) that names a constructor implicitly declares a set of forwarding constructors. For each non-template constructor other than a default or copy constructor in the class named in the using-declaration, a constructor is implicitly declared with the same parameter-type-list (8.3.5 dcl.fct), exception-specification (15.4 except.spec), absence or presence of explicit (7.1.2 dcl.fct.spec), and absence or presence of constexpr (7.1.6 dcl.constexpr), unless there is a user-declared constructor with the same signature in the class where the using-declaration appears. Similarly, for each constructor member function template in the class named in the using-declaration, a constructor member function template is implicitly declared with the same template parameter list (14.1 temp.param), parameter-type-list (8.3.5 dcl.fct), exception-specification (15.4 except.spec), absence or presence of explicit (7.1.2 dcl.fct.spec), and absence or presence of constexpr (7.1.6 dcl.constexpr) unless there is an equivalent user-declared constructor member function template (14.5.5.1 temp.over.link) in the class where the using-declaration appears. [Note: Default and copy constructors may be implicitly declared as specified in 12.1 class.ctor and 12.8 class.copy.]

[Note: If two using-dclarations declare forwarding constructors with the same signatures, the program is ill-formed (9.2 class.mem).

struct B1 {
  B1( int );
};

struct B2 {
  B2( int );
};

struct D1 : B1, B2 {
  using B1::B1;
  using B2::B2;  // ill-formed implicitly declaring same ctor twice
};

struct D2 : B1, B2 {
  using B1::B1;
  using B2::B2;
  D2( int );  // user declaration prevents conflict
};

]

An implicitly-declared forwarding constructor for a class is implicitly defined when it is used (3.2 basic.def.odr) to create an object of its class type (1.8 intro.object). An implicitly-defined forwarding constructor performs the set of initializations of the class that would be performed by a user-written inline constructor for that class with a mem-initializer-list whose only mem-initializer has a mem-initializer-id that names the base class named in the using-declaration and an expression-list as specified below, and with an empty function body (12.6.2 class.base.init). If that user-written constructor would be ill-formed, the program is ill-formed. Each expression in the expression-list is of the form static_cast<T&&>(p) where p is the name of the corresponding constructor parameter and T is the declared type of p.

[ Example:

struct B1 {
  B1( int ) {}
};

struct B2 {
  B2( double ) {}
};

struct D1 : B1 {
  using B1::B1; // implicitly declares D1( int )
  int x;
};

D1 d(6);        // ok; d1.x is not initialized
D1 e;           // error: base class B1 has no default constructor

struct D2 : B2 {
  using B2::B2; // ok, implicitly declares D2( double )
  B1 b;
};

D2 f(1.0);      // error: use of D2( double ) is ill-formed

template< class T >
struct D : T {
  using T::T;   // declares all constructors from class T
  ~D() { clog << "Destroying wrapper" << endl; }
};

Class template D wraps any class and forwards all its constructors, while writing a message to the standard log whenever an object of class D is destroyed.

]

While drafting the wording for this paper, it occurred to one of the authors that the standard currently defines "default constructor" in terms of whether a constructor can be called with no arguments. Thus, for a class C, C(int x = 42) is a default constructor. An implicitly declared default constructor could have that signature, which seems a bit surprising. Unrelated to the proposed semantics in this paper, we propose to change 12.1 class.ctor paragraph 5 as follows

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a default constructor with no arguments is implicitly declared. ...