12 Overloading [over]

12.2 Overload resolution [over.match]

12.2.2 Candidate functions and argument lists [over.match.funcs]

12.2.2.1 General [over.match.funcs.general]

1
#
The subclauses of [over.match.funcs] describe the set of candidate functions and the argument list submitted to overload resolution in each context in which overload resolution is used.
The source transformations and constructions defined in these subclauses are only for the purpose of describing the overload resolution process.
An implementation is not required to use such transformations and constructions.
2
#
The set of candidate functions can contain both member and non-member functions to be resolved against the same argument list.
If a member function is
  • (2.1)
    an implicit object member function that is not a constructor, or
  • (2.2)
    a static member function and the argument list includes an implied object argument,
it is considered to have an extra first parameter, called the implicit object parameter, which represents the object for which the member function has been called.
3
#
Similarly, when appropriate, the context can construct an argument list that contains an implied object argument as the first argument in the list to denote the object to be operated on.
4
#
For implicit object member functions, the type of the implicit object parameter is where X is the class of which the function is a direct member and cv is the cv-qualification on the member function declaration.
[Example 1: 
For a const member function of class X, the extra parameter is assumed to have type “lvalue reference to const X”.
— end example]
For conversion functions that are implicit object member functions, the function is considered to be a member of the class of the implied object argument for the purpose of defining the type of the implicit object parameter.
For non-conversion functions that are implicit object member functions nominated by a using-declaration in a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit object parameter.
For static member functions, the implicit object parameter is considered to match any object (since if the function is selected, the object is discarded).
[Note 1: 
No actual type is established for the implicit object parameter of a static member function, and no attempt will be made to determine a conversion sequence for that parameter ([over.match.best]).
— end note]
5
#
During overload resolution, the implied object argument is indistinguishable from other arguments.
The implicit object parameter, however, retains its identity since no user-defined conversions can be applied to achieve a type match with it.
For implicit object member functions declared without a ref-qualifier, even if the implicit object parameter is not const-qualified, an rvalue can be bound to the parameter as long as in all other respects the argument can be converted to the type of the implicit object parameter.
[Note 2: 
The fact that such an argument is an rvalue does not affect the ranking of implicit conversion sequences.
— end note]
6
#
Because other than in list-initialization only one user-defined conversion is allowed in an implicit conversion sequence, special rules apply when selecting the best user-defined conversion ([over.match.best], [over.best.ics]).
[Example 2: class T { public: T(); }; class C : T { public: C(int); }; T a = 1; // error: no viable conversion (T(C(1)) not considered) — end example]
7
#
In each case where conversion functions of a class S are considered for initializing an object or reference of type T, the candidate functions include the result of a search for the conversion-function-id operator T in S.
[Note 3: 
This search can find a specialization of a conversion function template ([basic.lookup]).
— end note]
Each such case also defines sets of permissible types for explicit and non-explicit conversion functions; each (non-template) conversion function that
  • (7.1)
    is a non-hidden member of S,
  • (7.2)
    yields a permissible type, and,
  • (7.3)
    for the former set, is non-explicit
is also a candidate function.
If initializing an object, for any permissible type cv U, any cv2 U, cv2 U&, or cv2 U&& is also a permissible type.
If the set of permissible types for explicit conversion functions is empty, any candidates that are explicit are discarded.
8
#
In each case where a candidate is a function template, candidate function template specializations are generated using template argument deduction ([temp.over], [temp.deduct]).
If a constructor template or conversion function template has an explicit-specifier whose constant-expression is value-dependent ([temp.dep]), template argument deduction is performed first and then, if the context admits only candidates that are not explicit and the generated specialization is explicit ([dcl.fct.spec]), it will be removed from the candidate set.
Those candidates are then handled as candidate functions in the usual way.101
A given name can refer to, or a conversion can consider, one or more function templates as well as a set of non-template functions.
In such a case, the candidate functions generated from each function template are combined with the set of non-template candidate functions.
9
#
A defaulted move special member function ([class.copy.ctor], [class.copy.assign]) that is defined as deleted is excluded from the set of candidate functions in all contexts.
A constructor inherited from class type C ([class.inhctor.init]) that has a first parameter of type “reference to cv1 P” (including such a constructor instantiated from a template) is excluded from the set of candidate functions when constructing an object of type cv2 D if the argument list has exactly one argument and C is reference-related to P and P is reference-related to D.
[Example 3: struct A { A(); // #1 A(A &&); // #2 template<typename T> A(T &&); // #3 }; struct B : A { using A::A; B(const B &); // #4 B(B &&) = default; // #5, implicitly deleted struct X { X(X &&) = delete; } x; }; extern B b1; B b2 = static_cast<B&&>(b1); // calls #4: #1 is not viable, #2, #3, and #5 are not candidates struct C { operator B&&(); }; B b3 = C(); // calls #4 — end example]
101)101)
The process of argument deduction fully determines the parameter types of the function template specializations, i.e., the parameters of function template specializations contain no template parameter types.
Therefore, except where specified otherwise, function template specializations and non-template functions ([dcl.fct]) are treated equivalently for the remainder of overload resolution.

12.2.2.2 Function call syntax [over.match.call]

12.2.2.2.1 General [over.match.call.general]

1
#
In a function call
postfix-expression ( expression-list )
if the postfix-expression names at least one function or function template, overload resolution is applied as specified in [over.call.func].
If the postfix-expression denotes an object of class type, overload resolution is applied as specified in [over.call.object].
2
#
If the postfix-expression is the address of an overload set, overload resolution is applied using that set as described above.
[Note 1: 
No implied object argument is added in this case.
— end note]
If the function selected by overload resolution is an implicit object member function, the program is ill-formed.
[Note 2: 
The resolution of the address of an overload set in other contexts is described in [over.over].
— end note]

12.2.2.2.2 Call to named function [over.call.func]

1
#
Of interest in [over.call.func] are only those function calls in which the postfix-expression ultimately contains an id-expression that denotes one or more functions.
Such a postfix-expression, perhaps nested arbitrarily deep in parentheses, has one of the following forms:
postfix-expression:
postfix-expression . id-expression
postfix-expression -> id-expression
primary-expression
These represent two syntactic subcategories of function calls: qualified function calls and unqualified function calls.
2
#
In qualified function calls, the function is named by an id-expression preceded by an -> or . operator.
Since the construct A->B is generally equivalent to (*A).B, the rest of [over] assumes, without loss of generality, that all member function calls have been normalized to the form that uses an object and the . operator.
Furthermore, [over] assumes that the postfix-expression that is the left operand of the . operator has type “cv T” where T denotes a class.102
The function declarations found by name lookup ([class.member.lookup]) constitute the set of candidate functions.
The argument list is the expression-list in the call augmented by the addition of the left operand of the . operator in the normalized member function call as the implied object argument ([over.match.funcs]).
3
#
In unqualified function calls, the function is named by a primary-expression.
The function declarations found by name lookup ([basic.lookup]) constitute the set of candidate functions.
Because of the rules for name lookup, the set of candidate functions consists either entirely of non-member functions or entirely of member functions of some class T.
In the former case or if the primary-expression is the address of an overload set, the argument list is the same as the expression-list in the call.
Otherwise, the argument list is the expression-list in the call augmented by the addition of an implied object argument as in a qualified function call.
If the current class is, or is derived from, T, and the keyword this ([expr.prim.this]) refers to it, then the implied object argument is (*this).
Otherwise, a contrived object of type T becomes the implied object argument;103 if overload resolution selects a non-static member function, the call is ill-formed.
[Example 1: struct C { void a(); void b() { a(); // OK, (*this).a() } void c(this const C&); // #1 void c() &; // #2 static void c(int = 0); // #3 void d() { c(); // error: ambiguous between #2 and #3 (C::c)(); // error: as above (&(C::c))(); // error: cannot resolve address of overloaded this->C​::​c ([over.over]) (&C::c)(C{}); // selects #1 (&C::c)(*this); // error: selects #2, and is ill-formed ([over.match.call.general]) (&C::c)(); // selects #3 } void f(this const C&); void g() const { f(); // OK, (*this).f() f(*this); // error: no viable candidate for (*this).f(*this) this->f(); // OK } static void h() { f(); // error: contrived object argument, but overload resolution // picked a non-static member function f(C{}); // error: no viable candidate C{}.f(); // OK } void k(this int); operator int() const; void m(this const C& c) { c.k(); // OK } }; — end example]
102)102)
Note that cv-qualifiers on the type of objects are significant in overload resolution for both glvalue and class prvalue objects.
103)103)
An implied object argument is contrived to correspond to the implicit object parameter attributed to member functions during overload resolution.
It is not used in the call to the selected function.
Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.

12.2.2.2.3 Call to object of class type [over.call.object]

1
#
If the postfix-expression E in the function call syntax evaluates to a class object of type “cv T”, then the set of candidate functions includes at least the function call operators of T.
The function call operators of T are the results of a search for the name operator() in the scope of T.
2
#
In addition, for each non-explicit conversion function declared in T of the form
operator conversion-type-id ( ) cv-qualifier-seq ref-qualifier noexcept-specifier attribute-specifier-seq ;
where the optional cv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes the type “pointer to function of () returning R”, or the type “reference to pointer to function of () returning R”, or the type “reference to function of () returning R”, a surrogate call function with the unique name call-function and having the form
R call-function ( conversion-type-id  F, P a, …, P a) { return F (a, …, a); }
is also considered as a candidate function.
Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration.104
3
#
The argument list submitted to overload resolution consists of the argument expressions present in the function call syntax preceded by the implied object argument (E).
[Note 1: 
When comparing the call against the function call operators, the implied object argument is compared against the object parameter of the function call operator.
When comparing the call against a surrogate call function, the implied object argument is compared against the first parameter of the surrogate call function.
— end note]
[Example 1: int f1(int); int f2(float); typedef int (*fp1)(int); typedef int (*fp2)(float); struct A { operator fp1() { return f1; } operator fp2() { return f2; } } a; int i = a(1); // calls f1 via pointer returned from conversion function — end example]
104)104)
Note that this construction can yield candidate call functions that cannot be differentiated one from the other by overload resolution because they have identical declarations or differ only in their return type.
The call will be ambiguous if overload resolution cannot select a match to the call that is uniquely better than such undifferentiable functions.

12.2.2.3 Operators in expressions [over.match.oper]

1
#
If no operand of an operator in an expression has a type that is a class or an enumeration, the operator is assumed to be a built-in operator and interpreted according to [expr.compound].
[Note 1: 
Because ., .*, and ​::​ cannot be overloaded, these operators are always built-in operators interpreted according to [expr.compound].
?: cannot be overloaded, but the rules in this subclause are used to determine the conversions to be applied to the second and third operands when they have class or enumeration type ([expr.cond]).
— end note]
[Example 1: struct String { String (const String&); String (const char*); operator const char* (); }; String operator + (const String&, const String&); void f() { const char* p= "one" + "two"; // error: cannot add two pointers; overloaded operator+ not considered // because neither operand has class or enumeration type int I = 1 + 1; // always evaluates to 2 even if class or enumeration types exist // that would perform the operation. } — end example]
2
#
If either operand has a type that is a class or an enumeration, a user-defined operator function can be declared that implements this operator or a user-defined conversion can be necessary to convert the operand to a type that is appropriate for a built-in operator.
In this case, overload resolution is used to determine which operator function or built-in operator is to be invoked to implement the operator.
Therefore, the operator notation is first transformed to the equivalent function-call notation as summarized in Table 18 (where @ denotes one of the operators covered in the specified subclause).
However, the operands are sequenced in the order prescribed for the built-in operator ([expr.compound]).
Table 18: Relationship between operator and function call notation [tab:over.match.oper]
Subclause
Expression
As member function
As non-member function
@a
(a).operator@ ( )
operator@(a)
a@b
(a).operator@ (b)
operator@(a, b)
a=b
(a).operator= (b)
a[b]
(a).operator[](b)
a->
(a).operator->( )
a@
(a).operator@ (0)
operator@(a, 0)
3
#
For a unary operator @ with an operand of type cv1 T1, and for a binary operator @ with a left operand of type cv1 T1 and a right operand of type cv2 T2, four sets of candidate functions, designated member candidates, non-member candidates, built-in candidates, and rewritten candidates, are constructed as follows:
  • (3.1)
    If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of a search for operator@ in the scope of T1; otherwise, the set of member candidates is empty.
  • (3.2)
    For the operators =, [], or ->, the set of non-member candidates is empty; otherwise, it includes the result of unqualified lookup for operator@ in the rewritten function call ([basic.lookup.unqual], [basic.lookup.argdep]), ignoring all member functions.
    However, if no operand has a class type, only those non-member functions in the lookup set that have a first parameter of type T1 or “reference to cv T1”, when T1 is an enumeration type, or (if there is a right operand) a second parameter of type T2 or “reference to cv T2”, when T2 is an enumeration type, are candidate functions.
  • (3.3)
    For the operator ,, the unary operator &, or the operator ->, the built-in candidates set is empty.
    For all other operators, the built-in candidates include all of the candidate operator functions defined in [over.built] that, compared to the given operator,
    • (3.3.1)
      have the same operator name, and
    • (3.3.2)
      accept the same number of operands, and
    • (3.3.3)
      accept operand types to which the given operand or operands can be converted according to [over.best.ics], and
    • (3.3.4)
      do not have the same parameter-type-list as any non-member candidate or rewritten non-member candidate that is not a function template specialization.
  • (3.4)
    The rewritten candidate set is determined as follows:
    • (3.4.1)
      For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x <=> y.
    • (3.4.2)
      For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y <=> x.
    • (3.4.3)
      For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y that are rewrite targets with first operand x (see below).
    • (3.4.4)
      For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x that is a rewrite target with first operand y.
    • (3.4.5)
      For all other operators, the rewritten candidate set is empty.
    [Note 2: 
    A candidate synthesized from a member candidate has its object parameter as the second parameter, thus implicit conversions are considered for the first, but not for the second, parameter.
    — end note]
4
#
A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression finds a function or function template that would correspond ([basic.scope.scope]) to F if its name were operator==, where S is the scope of the class type of o if F is a class member, and the namespace scope of which F is a member otherwise.
A function template specialization named operator== is a rewrite target if its function template is a rewrite target.
[Example 2: struct A {}; template<typename T> bool operator==(A, T); // #1 bool a1 = 0 == A(); // OK, calls reversed #1 template<typename T> bool operator!=(A, T); bool a2 = 0 == A(); // error, #1 is not a rewrite target struct B { bool operator==(const B&); // #2 }; struct C : B { C(); C(B); bool operator!=(const B&); // #3 }; bool c1 = B() == C(); // OK, calls #2; reversed #2 is not a candidate // because search for operator!= in C finds #3 bool c2 = C() == B(); // error: ambiguous between #2 found when searching C and // reversed #2 found when searching B struct D {}; template<typename T> bool operator==(D, T); // #4 inline namespace N { template<typename T> bool operator!=(D, T); // #5 } bool d1 = 0 == D(); // OK, calls reversed #4; #5 does not forbid #4 as a rewrite target — end example]
5
#
For the first parameter of the built-in assignment operators, only standard conversion sequences ([over.ics.scs]) are considered.
6
#
For all other operators, no such restrictions apply.
7
#
The set of candidate functions for overload resolution for some operator @ is the union of the member candidates, the non-member candidates, the built-in candidates, and the rewritten candidates for that operator @.
8
#
The argument list contains all of the operands of the operator.
The best function from the set of candidate functions is selected according to [over.match.viable] and [over.match.best].105
[Example 3: struct A { operator int(); }; A operator+(const A&, const A&); void m() { A a, b; a + b; // operator+(a, b) chosen over int(a) + int(b) } — end example]
9
#
If a rewritten operator<=> candidate is selected by overload resolution for an operator @, x @ y is interpreted as 0 @ (y <=> x) if the selected candidate is a synthesized candidate with reversed order of parameters, or (x <=> y) @ 0 otherwise, using the selected rewritten operator<=> candidate.
Rewritten candidates for the operator @ are not considered in the context of the resulting expression.
10
#
If a rewritten operator== candidate is selected by overload resolution for an operator @, its return type shall be cv bool, and x @ y is interpreted as:
  • (10.1)
    if @ is != and the selected candidate is a synthesized candidate with reversed order of parameters, !(y == x),
  • (10.2)
    otherwise, if @ is !=, !(x == y),
  • (10.3)
    otherwise (when @ is ==), y == x,
in each case using the selected rewritten operator== candidate.
11
#
If a built-in candidate is selected by overload resolution, the operands of class type are converted to the types of the corresponding parameters of the selected operation function, except that the second standard conversion sequence of a user-defined conversion sequence is not applied.
Then the operator is treated as the corresponding built-in operator and interpreted according to [expr.compound].
[Example 4: struct X { operator double(); }; struct Y { operator int*(); }; int *a = Y() + 100.0; // error: pointer arithmetic requires integral operand int *b = Y() + X(); // error: pointer arithmetic requires integral operand — end example]
12
#
The second operand of operator -> is ignored in selecting an operator-> function, and is not an argument when the operator-> function is called.
When operator-> returns, the operator -> is applied to the value returned, with the original second operand.106
13
#
If the operator is the operator ,, the unary operator &, or the operator ->, and there are no viable functions, then the operator is assumed to be the built-in operator and interpreted according to [expr.compound].
14
#
[Note 3: 
The lookup rules for operators in expressions are different than the lookup rules for operator function names in a function call, as shown in the following example: struct A { }; void operator + (A, A); struct B { void operator + (B); void f (); }; A a; void B::f() { operator+ (a,a); // error: global operator hidden by member a + a; // OK, calls global operator+ }
— end note]
105)105)
If the set of candidate functions is empty, overload resolution is unsuccessful.
106)106)
If the value returned by the operator-> function has class type, this can result in selecting and calling another operator-> function.
The process repeats until an operator-> function returns a value of non-class type.

12.2.2.4 Initialization by constructor [over.match.ctor]

1
#
When objects of class type are direct-initialized, copy-initialized from an expression of the same or a derived class type ([dcl.init]), or default-initialized, overload resolution selects the constructor.
For direct-initialization or default-initialization (including default-initialization in the context of copy-list-initialization), the candidate functions are all the constructors of the class of the object being initialized.
Otherwise, the candidate functions are all the converting constructors ([class.conv.ctor]) of that class.
The argument list is the expression-list or assignment-expression of the initializer.
For default-initialization in the context of copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.

12.2.2.5 Copy-initialization of class by user-defined conversion [over.match.copy]

1
#
Under the conditions specified in [dcl.init], as part of a copy-initialization of an object of class type, a user-defined conversion can be invoked to convert an initializer expression to the type of the object being initialized.
Overload resolution is used to select the user-defined conversion to be invoked.
[Note 1: 
The conversion performed for indirect binding to a reference to a possibly cv-qualified class type is determined in terms of a corresponding non-reference copy-initialization.
— end note]
Assuming that “cv1 T” is the type of the object being initialized, with T a class type, the candidate functions are selected as follows:
  • (1.1)
    The converting constructors of T are candidate functions.
  • (1.2)
    When the type of the initializer expression is a class type “cv S”, conversion functions are considered.
    The permissible types for non-explicit conversion functions are T and any class derived from T.
    When initializing a temporary object ([class.mem]) to be bound to the first parameter of a constructor where the parameter is of type “reference to cv2 T” and the constructor is called with a single argument in the context of direct-initialization of an object of type “cv3 T”, the permissible types for explicit conversion functions are the same; otherwise there are none.
2
#
In both cases, the argument list has one argument, which is the initializer expression.
[Note 2: 
This argument will be compared against the first parameter of the constructors and against the object parameter of the conversion functions.
— end note]

12.2.2.6 Initialization by conversion function [over.match.conv]

1
#
Under the conditions specified in [dcl.init], as part of an initialization of an object of non-class type, a conversion function can be invoked to convert an initializer expression of class type to the type of the object being initialized.
Overload resolution is used to select the conversion function to be invoked.
Assuming that “cv T” is the type of the object being initialized, the candidate functions are selected as follows:
  • (1.1)
    The permissible types for non-explicit conversion functions are those that can be converted to type T via a standard conversion sequence ([over.ics.scs]).
    For direct-initialization, the permissible types for explicit conversion functions are those that can be converted to type T with a (possibly trivial) qualification conversion ([conv.qual]); otherwise there are none.
2
#
The argument list has one argument, which is the initializer expression.
[Note 1: 
This argument will be compared against the object parameter of the conversion functions.
— end note]

12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]

1
#
Under the conditions specified in [dcl.init.ref], a reference can be bound directly to the result of applying a conversion function to an initializer expression.
Overload resolution is used to select the conversion function to be invoked.
Assuming that “reference to cv1 T” is the type of the reference being initialized, the candidate functions are selected as follows:
  • (1.1)
    Let R be a set of types including
    • (1.1.1)
      “lvalue reference to cv2 T2” (when initializing an lvalue reference or an rvalue reference to function) and
    • (1.1.2)
      “cv2 T2” and “rvalue reference to cv2 T2” (when initializing an rvalue reference or an lvalue reference to function)
    for any T2.
    The permissible types for non-explicit conversion functions are the members of R where “cv1 T” is reference-compatible ([dcl.init.ref]) with “cv2 T2”.
    For direct-initialization, the permissible types for explicit conversion functions are the members of R where T2 can be converted to type T with a (possibly trivial) qualification conversion ([conv.qual]); otherwise there are none.
2
#
The argument list has one argument, which is the initializer expression.
[Note 1: 
This argument will be compared against the object parameter of the conversion functions.
— end note]

12.2.2.8 Initialization by list-initialization [over.match.list]

1
#
When objects of non-aggregate class type T are list-initialized such that [dcl.init.list] specifies that overload resolution is performed according to the rules in this subclause or when forming a list-initialization sequence according to [over.ics.list], overload resolution selects the constructor in two phases:
  • (1.1)
    If the initializer list is not empty or T has no default constructor, overload resolution is first performed where the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class T and the argument list consists of the initializer list as a single argument.
  • (1.2)
    Otherwise, or if no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.
In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.
[Note 1: 
This differs from other situations ([over.match.ctor], [over.match.copy]), where only converting constructors are considered for copy-initialization.
This restriction only applies if this initialization is part of the final result of overload resolution.
— end note]

12.2.2.9 Class template argument deduction [over.match.class.deduct]

1
#
When resolving a placeholder for a deduced class type ([dcl.type.class.deduct]) where the template-name names a primary class template C, a set of functions and function templates, called the guides of C, is formed comprising:
In addition, if C is defined and its definition satisfies the conditions for an aggregate class ([dcl.init.aggr]) with the assumption that any dependent base class has no virtual functions and no virtual base classes, and the initializer is a non-empty braced-init-list or parenthesized expression-list, and there are no deduction-guides for C, the set contains an additional function template, called the aggregate deduction candidate, defined as follows.
Let be the elements of the initializer-list or designated-initializer-list of the braced-init-list, or of the expression-list.
For each , let be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by ([dcl.init.aggr]) if
  • (1.5)
    brace elision is not considered for any aggregate element that has
    • (1.5.1)
      a dependent non-array type,
    • (1.5.2)
      an array type with a value-dependent bound, or
    • (1.5.3)
      an array type with a dependent array element type and is a string literal; and
  • (1.6)
    each non-trailing aggregate element that is a pack expansion is assumed to correspond to no elements of the initializer list, and
  • (1.7)
    a trailing aggregate element that is a pack expansion is assumed to correspond to all remaining elements of the initializer list (if any).
If there is no such aggregate element for any , the aggregate deduction candidate is not added to the set.
The aggregate deduction candidate is derived as above from a hypothetical constructor , where
  • (1.8)
    if is of array type and is a braced-init-list, is an rvalue reference to the declared type of , and
  • (1.9)
    if is of array type and is a string-literal, is an lvalue reference to the const-qualified declared type of , and
  • (1.10)
    otherwise, is the declared type of ,
except that additional parameter packs of the form are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type that was skipped because it was a parameter pack, and the trailing sequence of parameters corresponding to a trailing aggregate element that is a pack expansion (if any) is replaced by a single parameter of the form .
In addition, if C is defined and inherits constructors ([namespace.udecl]) from a direct base class denoted in the base-specifier-list by a class-or-decltype B, let A be an alias template whose template parameter list is that of C and whose defining-type-id is B.
If A is a deducible template ([dcl.type.simple]), the set contains the guides of A with the return type R of each guide replaced with typename CC<R>​::​type given a class template template <typename> class CC; whose primary template is not defined and with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of A with the template argument list of A ([temp.dep.type]) having a member typedef type designating a template specialization with the template argument list of A but with C as the template.
[Note 2: 
Equivalently, the template parameter list of the specialization is that of C, the template argument list of the specialization is B, and the member typedef names C with the template argument list of C.
— end note]
2
#
[Example 1: template <typename T> struct B { B(T); }; template <typename T> struct C : public B<T> { using B<T>::B; }; template <typename T> struct D : public B<T> {}; C c(42); // OK, deduces C<int> D d(42); // error: deduction failed, no inherited deduction guides B(int) -> B<char>; C c2(42); // OK, deduces C<char> template <typename T> struct E : public B<int> { using B<int>::B; }; E e(42); // error: deduction failed, arguments of E cannot be deduced from introduced guides template <typename T, typename U, typename V> struct F { F(T, U, V); }; template <typename T, typename U> struct G : F<U, T, int> { using G::F::F; } G g(true, 'a', 1); // OK, deduces G<char, bool> template<class T, std::size_t N> struct H { T array[N]; }; template<class T, std::size_t N> struct I { volatile T array[N]; }; template<std::size_t N> struct J { unsigned char array[N]; }; H h = { "abc" }; // OK, deduces H<char, 4> (not T = const char) I i = { "def" }; // OK, deduces I<char, 4> J j = { "ghi" }; // error: cannot bind reference to array of unsigned char to array of char in deduction — end example]
3
#
When resolving a placeholder for a deduced class type ([dcl.type.simple]) where the template-name names an alias template A, the defining-type-id of A must be of the form
typename nested-name-specifier template simple-template-id
as specified in [dcl.type.simple].
The guides of A are the set of functions or function templates formed as follows.
For each function or function template f in the guides of the template named by the simple-template-id of the defining-type-id, the template arguments of the return type of f are deduced from the defining-type-id of A according to the process in [temp.deduct.type] with the exception that deduction does not fail if not all template arguments are deduced.
If deduction fails for another reason, proceed with an empty set of deduced template arguments.
Let g denote the result of substituting these deductions into f.
If substitution succeeds, form a function or function template f' with the following properties and add it to the set of guides of A:
  • (3.1)
    The function type of f' is the function type of g.
  • (3.2)
    If f is a function template, f' is a function template whose template parameter list consists of all the template parameters of A (including their default template arguments) that appear in the above deductions or (recursively) in their default template arguments, followed by the template parameters of f that were not deduced (including their default template arguments), otherwise f' is not a function template.
  • (3.3)
    The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of g and a constraint that is satisfied if and only if the arguments of A are deducible (see below) from the return type.
  • (3.4)
    If f is a copy deduction candidate, then f' is considered to be so as well.
  • (3.5)
    If f was generated from a deduction-guide ([temp.deduct.guide]), then f' is considered to be so as well.
  • (3.6)
    The explicit-specifier of f' is the explicit-specifier of g (if any).
4
#
The arguments of a template A are said to be deducible from a type T if, given a class template template <typename> class AA; with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of A with the template argument list of A ([temp.dep.type]), AA<T> matches the partial specialization.
5
#
Initialization and overload resolution are performed as described in [dcl.init] and [over.match.ctor], [over.match.copy], or [over.match.list] (as appropriate for the type of initialization performed) for an object of a hypothetical class type, where the guides of the template named by the placeholder are considered to be the constructors of that class type for the purpose of forming an overload set, and the initializer is provided by the context in which class template argument deduction was performed.
The following exceptions apply:
  • (5.1)
    The first phase in [over.match.list] (considering initializer-list constructors) is omitted if the initializer list consists of a single expression of type cv U, where U is, or is derived from, a specialization of the class template directly or indirectly named by the placeholder.
  • (5.2)
    During template argument deduction for the aggregate deduction candidate, the number of elements in a trailing parameter pack is only deduced from the number of remaining function arguments if it is not otherwise deduced.
If the function or function template was generated from a constructor or deduction-guide that had an explicit-specifier, each such notional constructor is considered to have that same explicit-specifier.
All such notional constructors are considered to be public members of the hypothetical class type.
6
#
[Example 2: template <class T> struct A { explicit A(const T&, ...) noexcept; // #1 A(T&&, ...); // #2 }; int i; A a1 = { i, i }; // error: explicit constructor #1 selected in copy-list-initialization during deduction, // cannot deduce from non-forwarding rvalue reference in #2 A a2{i, i}; // OK, #1 deduces to A<int> and also initializes A a3{0, i}; // OK, #2 deduces to A<int> and also initializes A a4 = {0, i}; // OK, #2 deduces to A<int> and also initializes template <class T> A(const T&, const T&) -> A<T&>; // #3 template <class T> explicit A(T&&, T&&) -> A<T>; // #4 A a5 = {0, 1}; // error: explicit deduction guide #4 selected in copy-list-initialization during deduction A a6{0,1}; // OK, #4 deduces to A<int> and #2 initializes A a7 = {0, i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructor A a8{0,i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructor template <class T> struct B { template <class U> using TA = T; template <class U> B(U, TA<U>); }; B b{(int*)0, (char*)0}; // OK, deduces B<char*> template <typename T> struct S { T x; T y; }; template <typename T> struct C { S<T> s; T t; }; template <typename T> struct D { S<int> s; T t; }; C c1 = {1, 2}; // error: deduction failed C c2 = {1, 2, 3}; // error: deduction failed C c3 = {{1u, 2u}, 3}; // OK, deduces C<int> D d1 = {1, 2}; // error: deduction failed D d2 = {1, 2, 3}; // OK, braces elided, deduces D<int> template <typename T> struct E { T t; decltype(t) t2; }; E e1 = {1, 2}; // OK, deduces E<int> template <typename... T> struct Types {}; template <typename... T> struct F : Types<T...>, T... {}; struct X {}; struct Y {}; struct Z {}; struct W { operator Y(); }; F f1 = {Types<X, Y, Z>{}, {}, {}}; // OK, F<X, Y, Z> deduced F f2 = {Types<X, Y, Z>{}, X{}, Y{}}; // OK, F<X, Y, Z> deduced F f3 = {Types<X, Y, Z>{}, X{}, W{}}; // error: conflicting types deduced; operator Y not considered — end example]
7
#
[Example 3: template <class T, class U> struct C { C(T, U); // #1 }; template<class T, class U> C(T, U) -> C<T, std::type_identity_t<U>>; // #2 template<class V> using A = C<V *, V *>; template<std::integral W> using B = A<W>; int i{}; double d{}; A a1(&i, &i); // deduces A<int> A a2(i, i); // error: cannot deduce V * from i A a3(&i, &d); // error: #1: cannot deduce (V*, V*) from (int *, double *) // #2: cannot deduce A<V> from C<int *, double *> B b1(&i, &i); // deduces B<int> B b2(&d, &d); // error: cannot deduce B<W> from C<double *, double *>
Possible exposition-only implementation of the above procedure: // The following concept ensures a specialization of A is deduced. template <class> class AA; template <class V> class AA<A<V>> { }; template <class T> concept deduces_A = requires { sizeof(AA<T>); }; // f1 is formed from the constructor #1 of C, generating the following function template template<class T, class U> auto f1(T, U) -> C<T, U>; // Deducing arguments for C<T, U> from C<V *, V*> deduces T as V * and U as V *; // f1' is obtained by transforming f1 as described by the above procedure. template<class V> requires deduces_A<C<V *, V *>> auto f1_prime(V *, V*) -> C<V *, V *>; // f2 is formed from the deduction-guide #2 of C template<class T, class U> auto f2(T, U) -> C<T, std::type_identity_t<U>>; // Deducing arguments for C<T, std​::​type_identity_t<U>> from C<V *, V*> deduces T as V *; // f2' is obtained by transforming f2 as described by the above procedure. template<class V, class U> requires deduces_A<C<V *, std::type_identity_t<U>>> auto f2_prime(V *, U) -> C<V *, std::type_identity_t<U>>; // The following concept ensures a specialization of B is deduced. template <class> class BB; template <class V> class BB<B<V>> { }; template <class T> concept deduces_B = requires { sizeof(BB<T>); }; // The guides for B derived from the above f1' and f2' for A are as follows: template<std::integral W> requires deduces_A<C<W *, W *>> && deduces_B<C<W *, W *>> auto f1_prime_for_B(W *, W *) -> C<W *, W *>; template<std::integral W, class U> requires deduces_A<C<W *, std::type_identity_t<U>>> && deduces_B<C<W *, std::type_identity_t<U>>> auto f2_prime_for_B(W *, U) -> C<W *, std::type_identity_t<U>>;
— end example]