12 Overloading [over]

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.

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.

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.

For implicit object member functions, the type of the implicit object parameter is

• (4.1)
  “lvalue reference to cv X” for functions declared without a ref-qualifier or with the & ref-qualifier
• (4.2)
  “rvalue reference to cv X” for functions declared with the && ref-qualifier

where X is the class of which the function is a direct member and cv is the cv-qualification on the member function declaration.

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).

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.

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]).

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.

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.

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.

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.

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].

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]).

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]

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.

For the first parameter of the built-in assignment operators, only standard conversion sequences ([over.ics.scs]) are considered.

For all other operators, no such restrictions apply.

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 @.

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

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.

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.

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].

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

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].

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.

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.

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.

In both cases, the argument list has one argument, which is the initializer expression.

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.

The argument list has one argument, which is the initializer expression.

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.

The argument list has one argument, which is the initializer expression.

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.

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:

• (1.1)
  If C is defined, for each constructor of C, a function template with the following properties:

  • (1.1.1)
    The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.
  • (1.1.2)
    The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of C and the associated constraints of the constructor, if any.

    [Note 1: 

    A constraint-expression in the template-head of C is checked for satisfaction before any constraints from the template-head or trailing requires-clause of the constructor.

    — end note]
  • (1.1.3)
    The parameter-declaration-clause is that of the constructor.
  • (1.1.4)
    The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.
• (1.2)
  If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().
• (1.3)
  An additional function template derived as above from a hypothetical constructor C(C), called the copy deduction candidate.
• (1.4)
  For each deduction-guide, a function or function template with the following properties:

  • (1.4.1)
    The template-head, if any, and parameter-declaration-clause are those of the deduction-guide.
  • (1.4.2)
    The return type is the simple-template-id of the deduction-guide.

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 $x_1,\dots ,x_n$ be the elements of the initializer-list or designated-initializer-list of the braced-init-list, or of the expression-list.

For each $x_i$, let $e_i$ be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by $x_i$ ([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 $x_i$ 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 $e_i$ for any $x_i$, the aggregate deduction candidate is not added to the set.

The aggregate deduction candidate is derived as above from a hypothetical constructor $\text{C}({\text{T}}_1,\dots ,{\text{T}}_n)$, where

• (1.8)
  if $e_i$ is of array type and $x_i$ is a braced-init-list, ${\text{T}}_i$ is an rvalue reference to the declared type of $e_i$, and
• (1.9)
  if $e_i$ is of array type and $x_i$ is a string-literal, ${\text{T}}_i$ is an lvalue reference to the const-qualified declared type of $e_i$, and
• (1.10)
  otherwise, ${\text{T}}_i$ is the declared type of $e_i$,

except that additional parameter packs of the form ${\text{P}}_j\text{...}$ are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type ${\text{P}}_j$ 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 ${\text{T}}_n\text{...}$.

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.

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 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).

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.

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.

Define ${\text{ICS}}^i\left(\text{F}\right)$ as the implicit conversion sequence that converts the $i^{\text{th}}$ argument in the list to the type of the $i^{\text{th}}$ parameter of viable function F.

[over.best.ics] defines the implicit conversion sequences and [over.ics.rank] defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.

Given these definitions, a viable function ${\text{F}}_1$ is defined to be a better function than another viable function ${\text{F}}_2$ if for all arguments i, ${\text{ICS}}^i\left({\text{F}}_1\right)$ is not a worse conversion sequence than ${\text{ICS}}^i\left({\text{F}}_2\right)$, and then

• (2.1)
  for some argument j, ${\text{ICS}}^j\left({\text{F}}_1\right)$ is a better conversion sequence than ${\text{ICS}}^j\left({\text{F}}_2\right)$, or, if not that,
• (2.2)
  the context is an initialization by user-defined conversion (see [dcl.init], [over.match.conv], and [over.match.ref]) and the standard conversion sequence from the result of ${\text{F}}_1$ to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the result of ${\text{F}}_2$ to the destination type
  [Example 1: struct A { A(); operator int(); operator double(); } a; int i = a; // a.operator int() followed by no conversion is better than // a.operator double() followed by a conversion to int float x = a; // ambiguous: both possibilities require conversions, // and neither is better than the other — end example]
  or, if not that,
• (2.3)
  the context is an initialization by conversion function for direct reference binding of a reference to function type, the return type of ${\text{F}}_1$ is the same kind of reference (lvalue or rvalue) as the reference being initialized, and the return type of ${\text{F}}_2$ is not
  [Example 2: template <class T> struct A { operator T&(); // #1 operator T&&(); // #2 }; typedef int Fn(); A<Fn> a; Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 — end example]
  or, if not that,
• (2.4)
  ${\text{F}}_1$ is not a function template specialization and ${\text{F}}_2$ is a function template specialization, or, if not that,
• (2.5)
  ${\text{F}}_1$ and ${\text{F}}_2$ are function template specializations, and the function template for ${\text{F}}_1$ is more specialized than the template for ${\text{F}}_2$ according to the partial ordering rules described in [temp.func.order], or, if not that,
• (2.6)
  ${\text{F}}_1$ and ${\text{F}}_2$ are non-template functions and
  • (2.6.1)
    they have the same non-object-parameter-type-lists ([dcl.fct]), and
  • (2.6.2)
    if they are member functions, both are direct members of the same class, and
  • (2.6.3)
    if both are non-static member functions, they have the same types for their object parameters, and
  • (2.6.4)
    ${\text{F}}_1$ is more constrained than ${\text{F}}_2$ according to the partial ordering of constraints described in [temp.constr.order]
  [Example 3: template <typename T = int> struct S { constexpr void f(); // #1 constexpr void f(this S&) requires true; // #2 }; void test() { S<> s; s.f(); // calls #2 } — end example]
  or, if not that,
• (2.7)
  ${\text{F}}_1$ is a constructor for a class D, ${\text{F}}_2$ is a constructor for a base class B of D, and for all arguments the corresponding parameters of ${\text{F}}_1$ and ${\text{F}}_2$ have the same type
  [Example 4: struct A { A(int = 0); }; struct B: A { using A::A; B(); }; int main() { B b; // OK, B​::​B() } — end example]
  or, if not that,
• (2.8)
  ${\text{F}}_2$ is a rewritten candidate ([over.match.oper]) and ${\text{F}}_1$ is not
  [Example 5: struct S { friend auto operator<=>(const S&, const S&) = default; // #1 friend bool operator<(const S&, const S&); // #2 }; bool b = S() < S(); // calls #2 — end example]
  or, if not that,
• (2.9)
  ${\text{F}}_1$ and ${\text{F}}_2$ are rewritten candidates, and ${\text{F}}_2$ is a synthesized candidate with reversed order of parameters and ${\text{F}}_1$ is not
  [Example 6: struct S { friend std::weak_ordering operator<=>(const S&, int); // #1 friend std::weak_ordering operator<=>(int, const S&); // #2 }; bool b = 1 < S(); // calls #2 — end example]
  or, if not that,
• (2.10)
  ${\text{F}}_1$ and ${\text{F}}_2$ are generated from class template argument deduction ([over.match.class.deduct]) for a class D, and ${\text{F}}_2$ is generated from inheriting constructors from a base class of D while ${\text{F}}_1$ is not, and for each explicit function argument, the corresponding parameters of ${\text{F}}_1$ and ${\text{F}}_2$ are either both ellipses or have the same type, or, if not that,
• (2.11)
  ${\text{F}}_1$ is generated from a deduction-guide ([over.match.class.deduct]) and ${\text{F}}_2$ is not, or, if not that,
• (2.12)
  ${\text{F}}_1$ is the copy deduction candidate and ${\text{F}}_2$ is not, or, if not that,
• (2.13)
  ${\text{F}}_1$ is generated from a non-template constructor and ${\text{F}}_2$ is generated from a constructor template.
  [Example 7: template <class T> struct A { using value_type = T; A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3 template<class U> A(int, T, U); // #4 // #5 is the copy deduction candidate, A(A) }; A x(1, 2, 3); // uses #3, generated from a non-template constructor template <class T> A(T) -> A<T>; // #6, less specialized than #5 A a(42); // uses #6 to deduce A<int> and #1 to initialize A b = a; // uses #5 to deduce A<int> and #2 to initialize template <class T> A(A<T>) -> A<A<T>>; // #7, as specialized as #5 A b2 = a; // uses #7 to deduce A<A<int>> and #1 to initialize — end example]

If there is exactly one viable function that is a better function than all other viable functions, then it is the one selected by overload resolution; otherwise the call is ill-formed.107

If the best viable function resolves to a function for which multiple declarations were found, and if any two of these declarations inhabit different scopes and specify a default argument that made the function viable, the program is ill-formed.

This subclause defines a partial ordering of implicit conversion sequences based on the relationships better conversion sequence and better conversion.

If an implicit conversion sequence S1 is defined by these rules to be a better conversion sequence than S2, then it is also the case that S2 is a worse conversion sequence than S1.

If conversion sequence S1 is neither better than nor worse than conversion sequence S2, S1 and S2 are said to be indistinguishable conversion sequences.

When comparing the basic forms of implicit conversion sequences (as defined in [over.best.ics])

• (2.1)
  a standard conversion sequence is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and
• (2.2)
  a user-defined conversion sequence is a better conversion sequence than an ellipsis conversion sequence.

Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

• (3.1)
  List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if

  • (3.1.1)
    L1 converts to std​::​initializer_list<X> for some X and L2 does not, or, if not that,
  • (3.1.2)
    L1 and L2 convert to arrays of the same element type, and either the number of elements $n_1$ initialized by L1 is less than the number of elements $n_2$ initialized by L2, or $n_1=n_2$ and L2 converts to an array of unknown bound and L1 does not,

  even if one of the other rules in this paragraph would otherwise apply.

  [Example 1: void f1(int); // #1 void f1(std::initializer_list<long>); // #2 void g1() { f1({42}); } // chooses #2 void f2(std::pair<const char*, const char*>); // #3 void f2(std::initializer_list<std::string>); // #4 void g2() { f2({"foo","bar"}); } // chooses #4 — end example]

  [Example 2: void f(int (&&)[] ); // #1 void f(double (&&)[] ); // #2 void f(int (&&)[2]); // #3 f( {1} ); // Calls #1: Better than #2 due to conversion, better than #3 due to bounds f( {1.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1.0, 2.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1, 2} ); // Calls #3: Converting to array of known bound is better than to unknown bound, // and an identity conversion is better than floating-integral conversion — end example]
• (3.2)
  Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if

  • (3.2.1)
    S1 is a proper subsequence of S2 (comparing the conversion sequences in the canonical form defined by [over.ics.scs], excluding any Lvalue Transformation; the identity conversion sequence is considered to be a subsequence of any non-identity conversion sequence) or, if not that,
  • (3.2.2)
    the rank of S1 is better than the rank of S2, or S1 and S2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,
  • (3.2.3)
    S1 and S2 include reference bindings ([dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference
    [Example 3: int i; int f1(); int&& f2(); int g(const int&); int g(const int&&); int j = g(i); // calls g(const int&) int k = g(f1()); // calls g(const int&&) int l = g(f2()); // calls g(const int&&) struct A { A& operator<<(int); void p() &; void p() &&; }; A& operator<<(A&&, char); A() << 1; // calls A​::​operator<<(int) A() << 'c'; // calls operator<<(A&&, char) A a; a << 1; // calls A​::​operator<<(int) a << 'c'; // calls A​::​operator<<(int) A().p(); // calls A​::​p()&& a.p(); // calls A​::​p()& — end example]
    or, if not that,
  • (3.2.4)
    S1 and S2 include reference bindings ([dcl.init.ref]) and S1 binds an lvalue reference to an lvalue of function type and S2 binds an rvalue reference to an lvalue of function type
    [Example 4: int f(void(&)()); // #1 int f(void(&&)()); // #2 void g(); int i1 = f(g); // calls #1 — end example]
    or, if not that,
  • (3.2.5)
    S1 and S2 differ only in their qualification conversion ([conv.qual]) and yield similar types T1 and T2, respectively (where a standard conversion sequence that is a reference binding is considered to yield the cv-unqualified referenced type), where T1 and T2 are not the same type, and const T2 is reference-compatible with T1 ([dcl.init.ref])
    [Example 5: int f(const volatile int *); int f(const int *); int i; int j = f(&i); // calls f(const int*) int g(const int*); int g(const volatile int* const&); int* p; int k = g(p); // calls g(const int*) — end example]
    or, if not that,
  • (3.2.6)
    S1 and S2 bind “reference to T1” and “reference to T2”, respectively ([dcl.init.ref]), where T1 and T2 are not the same type, and T2 is reference-compatible with T1
    [Example 6: int f(const int &); int f(int &); int g(const int &); int g(int); int i; int j = f(i); // calls f(int &) int k = g(i); // ambiguous struct X { void f() const; void f(); }; void g(const X& a, X b) { a.f(); // calls X​::​f() const b.f(); // calls X​::​f() } int h1(int (&)[]); int h1(int (&)[1]); int h2(void (&)()); int h2(void (&)() noexcept); void g2() { int a[1]; h1(a); // calls h1(int (&)[1]) extern void f2() noexcept; h2(f2); // calls h2(void (&)() noexcept) } — end example]
    or, if not that,
  • (3.2.7)
    S1 and S2 bind the same reference type “reference to T” and have source types V1 and V2, respectively, where the standard conversion sequence from V1* to T* is better than the standard conversion sequence from V2* to T*.
    [Example 7: struct Z {}; struct A { operator Z&(); operator const Z&(); // #1 }; struct B { operator Z(); operator const Z&&(); // #2 }; const Z& r1 = A(); // OK, uses #1 const Z&& r2 = B(); // OK, uses #2 — end example]
• (3.3)
  User-defined conversion sequence U1 is a better conversion sequence than another user-defined conversion sequence U2 if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence of U1 is better than the second standard conversion sequence of U2.

  [Example 8: struct A { operator short(); } a; int f(int); int f(float); int i = f(a); // calls f(int), because short → int is // better than short → float. — end example]

Standard conversion sequences are ordered by their ranks: an Exact Match is a better conversion than a Promotion, which is a better conversion than a Conversion.

Two conversion sequences with the same rank are indistinguishable unless one of the following rules applies:

• (4.1)
  A conversion that does not convert a pointer or a pointer to member to bool is better than one that does.
• (4.2)
  A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.
• (4.3)
  A conversion in either direction between floating-point type FP1 and floating-point type FP2 is better than a conversion in the same direction between FP1 and arithmetic type T3 if

  • (4.3.1)
    the floating-point conversion rank ([conv.rank]) of FP1 is equal to the rank of FP2, and
  • (4.3.2)
    T3 is not a floating-point type, or T3 is a floating-point type whose rank is not equal to the rank of FP1, or the floating-point conversion subrank ([conv.rank]) of FP2 is greater than the subrank of T3.
    [Example 9: int f(std::float32_t); int f(std::float64_t); int f(long long); float x; std::float16_t y; int i = f(x); // calls f(std​::​float32_t) on implementations where // float and std​::​float32_t have equal conversion ranks int j = f(y); // error: ambiguous, no equal conversion rank — end example]
• (4.4)
  If class B is derived directly or indirectly from class A, conversion of B* to A* is better than conversion of B* to void*, and conversion of A* to void* is better than conversion of B* to void*.
• (4.5)
  If class B is derived directly or indirectly from class A and class C is derived directly or indirectly from B,

  • (4.5.1)
    conversion of C* to B* is better than conversion of C* to A*,
    [Example 10: struct A {}; struct B : public A {}; struct C : public B {}; C* pc; int f(A*); int f(B*); int i = f(pc); // calls f(B*) — end example]
  • (4.5.2)
    binding of an expression of type C to a reference to type B is better than binding an expression of type C to a reference to type A,
  • (4.5.3)
    conversion of A​::​* to B​::​* is better than conversion of A​::​* to C​::​*,
  • (4.5.4)
    conversion of C to B is better than conversion of C to A,
  • (4.5.5)
    conversion of B* to A* is better than conversion of C* to A*,
  • (4.5.6)
    binding of an expression of type B to a reference to type A is better than binding an expression of type C to a reference to type A,
  • (4.5.7)
    conversion of B​::​* to C​::​* is better than conversion of A​::​* to C​::​*, and
  • (4.5.8)
    conversion of B to A is better than conversion of C to A.
  [Note 1: 

  Compared conversion sequences will have different source types only in the context of comparing the second standard conversion sequence of an initialization by user-defined conversion (see [over.match.best]); in all other contexts, the source types will be the same and the target types will be different.

  — end note]