12 Overloading [over]

12.2 Overload resolution [over.match]

12.2.4 Best viable function [over.match.best]

12.2.4.1 General [over.match.best.general]

1
#
Define as the implicit conversion sequence that converts the argument in the list to the type of the 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.
2
#
Given these definitions, a viable function is defined to be a better function than another viable function if for all arguments i, is not a worse conversion sequence than , and then
  • (2.1)
    for some argument j, is a better conversion sequence than , 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 return type of 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 return type of 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 F1 is the same kind of reference (lvalue or rvalue) as the reference being initialized, and the return type of F2 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)
    F1 is not a function template specialization and F2 is a function template specialization, or, if not that,
  • (2.5)
    F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in [temp.func.order], or, if not that,
  • (2.6)
    F1 and F2 are non-template functions with the same parameter-type-lists, and F1 is more constrained than F2 according to the partial ordering of constraints described in [temp.constr.order], or if not that,
  • (2.7)
    F1 is a constructor for a class D, F2 is a constructor for a base class B of D, and for all arguments the corresponding parameters of F1 and F2 have the same type
    [Example 3: 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)
    F2 is a rewritten candidate ([over.match.oper]) and F1 is not
    [Example 4: 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)
    F1 and F2 are rewritten candidates, and F2 is a synthesized candidate with reversed order of parameters and F1 is not
    [Example 5: 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)
    F1 and F2 are generated from class template argument deduction ([over.match.class.deduct]) for a class D, and F2 is generated from inheriting constructors from a base class of D while F1 is not, and for each explicit function argument, the corresponding parameters of F1 and F2 are either both ellipses or have the same type, or, if not that,
  • (2.11)
    F1 is generated from a deduction-guide ([over.match.class.deduct]) and F2 is not, or, if not that,
  • (2.12)
    F1 is the copy deduction candidate and F2 is not, or, if not that,
  • (2.13)
    F1 is generated from a non-template constructor and F2 is generated from a constructor template.
    [Example 6: 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]
3
#
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.112
[Example 7: void Fcn(const int*, short); void Fcn(int*, int); int i; short s = 0; void f() { Fcn(&i, s); // is ambiguous because &i → int* is better than &i → const int* // but s → short is also better than s → int Fcn(&i, 1L); // calls Fcn(int*, int), because &i → int* is better than &i → const int* // and 1L → short and 1L → int are indistinguishable Fcn(&i, 'c'); // calls Fcn(int*, int), because &i → int* is better than &i → const int* // and 'c' → int is better than 'c' → short } — end example]
4
#
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.
[Example 8: namespace A { extern "C" void f(int = 5); } namespace B { extern "C" void f(int = 5); } using A::f; using B::f; void use() { f(3); // OK, default argument was not used for viability f(); // error: found default argument twice } — end example]
112)112)
The algorithm for selecting the best viable function is linear in the number of viable functions.
Run a simple tournament to find a function W that is not worse than any opponent it faced.
Although it is possible that another function F that W did not face is at least as good as W, F cannot be the best function because at some point in the tournament F encountered another function G such that F was not better than G.
Hence, either W is the best function or there is no best function.
So, make a second pass over the viable functions to verify that W is better than all other functions.

12.2.4.2 Implicit conversion sequences [over.best.ics]

12.2.4.2.1 General [over.best.ics.general]

1
#
An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called.
The sequence of conversions is an implicit conversion as defined in [conv], which means it is governed by the rules for initialization of an object or reference by a single expression ([dcl.init], [dcl.init.ref]).
2
#
Implicit conversion sequences are concerned only with the type, cv-qualification, and value category of the argument and how these are converted to match the corresponding properties of the parameter.
[Note 1: 
Other properties, such as the lifetime, storage class, alignment, accessibility of the argument, whether the argument is a bit-field, and whether a function is deleted, are ignored.
So, although an implicit conversion sequence can be defined for a given argument-parameter pair, the conversion from the argument to the parameter might still be ill-formed in the final analysis.
— end note]
3
#
A well-formed implicit conversion sequence is one of the following forms:
4
#
However, if the target is
  • (4.1)
    the first parameter of a constructor or
  • (4.2)
    the object parameter of a user-defined conversion function
and the constructor or user-defined conversion function is a candidate by user-defined conversion sequences are not considered.
[Note 2: 
These rules prevent more than one user-defined conversion from being applied during overload resolution, thereby avoiding infinite recursion.
— end note]
[Example 1: struct Y { Y(int); }; struct A { operator int(); }; Y y1 = A(); // error: A​::​operator int() is not a candidate struct X { X(); }; struct B { operator X(); }; B b; X x{{b}}; // error: B​::​operator X() is not a candidate — end example]
5
#
For the case where the parameter type is a reference, see [over.ics.ref].
6
#
When the parameter type is not a reference, the implicit conversion sequence models a copy-initialization of the parameter from the argument expression.
The implicit conversion sequence is the one required to convert the argument expression to a prvalue of the type of the parameter.
[Note 3: 
When the parameter has a class type, this is a conceptual conversion defined for the purposes of [over]; the actual initialization is defined in terms of constructors and is not a conversion.
— end note]
Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.
[Example 2: 
A parameter of type A can be initialized from an argument of type const A.
The implicit conversion sequence for that case is the identity sequence; it contains no “conversion” from const A to A.
— end example]
7
#
When the parameter has a class type and the argument expression has the same type, the implicit conversion sequence is an identity conversion.
When the parameter has a class type and the argument expression has a derived class type, the implicit conversion sequence is a derived-to-base conversion from the derived class to the base class.
A derived-to-base conversion has Conversion rank ([over.ics.scs]).
[Note 4: 
There is no such standard conversion; this derived-to-base conversion exists only in the description of implicit conversion sequences.
— end note]
8
#
When the parameter is the implicit object parameter of a static member function, the implicit conversion sequence is a standard conversion sequence that is neither better nor worse than any other standard conversion sequence.
9
#
In all contexts, when converting to the implicit object parameter or when converting to the left operand of an assignment operation only standard conversion sequences are allowed.
[Note 5: 
When converting to the explicit object parameter, if any, user-defined conversion sequences are allowed.
— end note]
10
#
If no conversions are required to match an argument to a parameter type, the implicit conversion sequence is the standard conversion sequence consisting of the identity conversion ([over.ics.scs]).
11
#
If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.
12
#
If there are multiple well-formed implicit conversion sequences converting the argument to the parameter type, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated the ambiguous conversion sequence.
For the purpose of ranking implicit conversion sequences as described in [over.ics.rank], the ambiguous conversion sequence is treated as a user-defined conversion sequence that is indistinguishable from any other user-defined conversion sequence.
[Note 6: 
This rule prevents a function from becoming non-viable because of an ambiguous conversion sequence for one of its parameters.
[Example 3: class B; class A { A (B&);}; class B { operator A (); }; class C { C (B&); }; void f(A) { } void f(C) { } B b; f(b); // error: ambiguous because there is a conversion b → C (via constructor) // and an (ambiguous) conversion b → A (via constructor or conversion function) void f(B) { } f(b); // OK, unambiguous — end example]
— end note]
If a function that uses the ambiguous conversion sequence is selected as the best viable function, the call will be ill-formed because the conversion of one of the arguments in the call is ambiguous.
13
#
The three forms of implicit conversion sequences mentioned above are defined in the following subclauses.

12.2.4.2.2 Standard conversion sequences [over.ics.scs]

1
#
Table 19 summarizes the conversions defined in [conv] and partitions them into four disjoint categories: Lvalue Transformation, Qualification Adjustment, Promotion, and Conversion.
[Note 1: 
These categories are orthogonal with respect to value category, cv-qualification, and data representation: the Lvalue Transformations do not change the cv-qualification or data representation of the type; the Qualification Adjustments do not change the value category or data representation of the type; and the Promotions and Conversions do not change the value category or cv-qualification of the type.
— end note]
2
#
[Note 2: 
As described in [conv], a standard conversion sequence either is the Identity conversion by itself (that is, no conversion) or consists of one to three conversions from the other four categories.
If there are two or more conversions in the sequence, the conversions are applied in the canonical order: Lvalue Transformation, Promotion or Conversion, Qualification Adjustment.
— end note]
3
#
Each conversion in Table 19 also has an associated rank (Exact Match, Promotion, or Conversion).
The rank of a conversion sequence is determined by considering the rank of each conversion in the sequence and the rank of any reference binding.
If any of those has Conversion rank, the sequence has Conversion rank; otherwise, if any of those has Promotion rank, the sequence has Promotion rank; otherwise, the sequence has Exact Match rank.
Table 19: Conversions [tab:over.ics.scs]
Conversion
Category
Rank
Subclause
No conversions required
Identity
Lvalue-to-rvalue conversion
Array-to-pointer conversion
Lvalue Transformation
Function-to-pointer conversion
Exact Match
Qualification conversions
Function pointer conversion
Qualification Adjustment
Integral promotions
Floating-point promotion
Promotion
Promotion
Integral conversions
Floating-point conversions
Floating-integral conversions
Pointer conversions
Conversion
Conversion
Pointer-to-member conversions
Boolean conversions

12.2.4.2.3 User-defined conversion sequences [over.ics.user]

1
#
A user-defined conversion sequence consists of an initial standard conversion sequence followed by a user-defined conversion ([class.conv]) followed by a second standard conversion sequence.
If the user-defined conversion is specified by a constructor ([class.conv.ctor]), the initial standard conversion sequence converts the source type to the type of the first parameter of that constructor.
If the user-defined conversion is specified by a conversion function, the initial standard conversion sequence converts the source type to the type of the object parameter of that conversion function.
2
#
The second standard conversion sequence converts the result of the user-defined conversion to the target type for the sequence; any reference binding is included in the second standard conversion sequence.
Since an implicit conversion sequence is an initialization, the special rules for initialization by user-defined conversion apply when selecting the best user-defined conversion for a user-defined conversion sequence (see [over.match.best] and [over.best.ics]).
3
#
If the user-defined conversion is specified by a specialization of a conversion function template, the second standard conversion sequence shall have exact match rank.
4
#
A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a constructor (i.e., a user-defined conversion function) is called for those cases.

12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]

1
#
An ellipsis conversion sequence occurs when an argument in a function call is matched with the ellipsis parameter specification of the function called (see [expr.call]).

12.2.4.2.5 Reference binding [over.ics.ref]

1
#
When a parameter of reference type binds directly to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base conversion ([over.best.ics]).
[Example 1: struct A {}; struct B : public A {} b; int f(A&); int f(B&); int i = f(b); // calls f(B&), an exact match, rather than f(A&), a conversion — end example]
If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence ([over.ics.user]) whose second standard conversion sequence is either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base conversion.
2
#
When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the referenced type according to [over.best.ics].
Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the referenced type with the argument expression.
Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.
3
#
Except for an implicit object parameter, for which see [over.match.funcs], an implicit conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue other than a function lvalue.
[Note 1: 
This means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue reference parameter (other than the implicit object parameter) and the corresponding argument would require a temporary to be created to initialize the lvalue reference (see [dcl.init.ref]).
— end note]
4
#
Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of an implicit conversion sequence, however.
[Example 2: 
A function with an “lvalue reference to int” parameter can be a viable candidate even if the corresponding argument is an int bit-field.
The formation of implicit conversion sequences treats the int bit-field as an int lvalue and finds an exact match with the parameter.
If the function is selected by overload resolution, the call will nonetheless be ill-formed because of the prohibition on binding a non-const lvalue reference to a bit-field ([dcl.init.ref]).
— end example]

12.2.4.2.6 List-initialization sequence [over.ics.list]

1
#
When an argument is an initializer list ([dcl.init.list]), it is not an expression and special rules apply for converting it to a parameter type.
2
#
If the initializer list is a designated-initializer-list, a conversion is only possible if the parameter has an aggregate type that can be initialized from the initializer list according to the rules for aggregate initialization ([dcl.init.aggr]), in which case the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.
[Note 1: 
Aggregate initialization does not require that the members are declared in designation order.
If, after overload resolution, the order does not match for the selected overload, the initialization of the parameter will be ill-formed ([dcl.init.list]).
[Example 1: struct A { int x, y; }; struct B { int y, x; }; void f(A a, int); // #1 void f(B b, ...); // #2 void g(A a); // #3 void g(B b); // #4 void h() { f({.x = 1, .y = 2}, 0); // OK; calls #1 f({.y = 2, .x = 1}, 0); // error: selects #1, initialization of a fails // due to non-matching member order ([dcl.init.list]) g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4 } — end example]
— end note]
3
#
Otherwise, if the parameter type is an aggregate class X and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence is the one required to convert the element to the parameter type.
4
#
Otherwise, if the parameter type is a character array113 and the initializer list has a single element that is an appropriately-typed string-literal ([dcl.init.string]), the implicit conversion sequence is the identity conversion.
5
#
Otherwise, if the parameter type is std​::​initializer_list<X> and all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X, or if the initializer list has no elements, the identity conversion.
This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor.
[Example 2: void f(std::initializer_list<int>); f( {} ); // OK, f(initializer_list<int>) identity conversion f( {1,2,3} ); // OK, f(initializer_list<int>) identity conversion f( {'a','b'} ); // OK, f(initializer_list<int>) integral promotion f( {1.0} ); // error: narrowing struct A { A(std::initializer_list<double>); // #1 A(std::initializer_list<complex<double>>); // #2 A(std::initializer_list<std::string>); // #3 }; A a{ 1.0,2.0 }; // OK, uses #1 void g(A); g({ "foo", "bar" }); // OK, uses #3 typedef int IA[3]; void h(const IA&); h({ 1, 2, 3 }); // OK, identity conversion — end example]
6
#
Otherwise, if the parameter type is “array of N X” or “array of unknown bound of X”, if there exists an implicit conversion sequence from each element of the initializer list (and from {} in the former case if N exceeds the number of elements in the initializer list) to X, the implicit conversion sequence is the worst such implicit conversion sequence.
7
#
Otherwise, if the parameter is a non-aggregate class X and overload resolution per [over.match.list] chooses a single best constructor C of X to perform the initialization of an object of type X from the argument initializer list:
  • (7.1)
    If C is not an initializer-list constructor and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence has Exact Match rank if U is X, or Conversion rank if U is derived from X.
  • (7.2)
    Otherwise, the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.
If multiple constructors are viable but none is better than the others, the implicit conversion sequence is the ambiguous conversion sequence.
User-defined conversions are allowed for conversion of the initializer list elements to the constructor parameter types except as noted in [over.best.ics].
[Example 3: struct A { A(std::initializer_list<int>); }; void f(A); f( {'a', 'b'} ); // OK, f(A(std​::​initializer_list<int>)) user-defined conversion struct B { B(int, double); }; void g(B); g( {'a', 'b'} ); // OK, g(B(int, double)) user-defined conversion g( {1.0, 1.0} ); // error: narrowing void f(B); f( {'a', 'b'} ); // error: ambiguous f(A) or f(B) struct C { C(std::string); }; void h(C); h({"foo"}); // OK, h(C(std​::​string("foo"))) struct D { D(A, C); }; void i(D); i({ {1,2}, {"bar"} }); // OK, i(D(A(std​::​initializer_list<int>{1,2}), C(std​::​string("bar")))) — end example]
8
#
Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization ([dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.
[Example 4: struct A { int m1; double m2; }; void f(A); f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowing — end example]
9
#
Otherwise, if the parameter is a reference, see [over.ics.ref].
[Note 2: 
The rules in this subclause will apply for initializing the underlying temporary for the reference.
— end note]
[Example 5: struct A { int m1; double m2; }; void f(const A&); f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowing void g(const double &); g({1}); // same conversion as int to double — end example]
10
#
Otherwise, if the parameter type is not a class:
  • (10.1)
    if the initializer list has one element that is not itself an initializer list, the implicit conversion sequence is the one required to convert the element to the parameter type;
    [Example 6: void f(int); f( {'a'} ); // OK, same conversion as char to int f( {1.0} ); // error: narrowing — end example]
  • (10.2)
    if the initializer list has no elements, the implicit conversion sequence is the identity conversion.
    [Example 7: void f(int); f( { } ); // OK, identity conversion — end example]
11
#
In all cases other than those enumerated above, no conversion is possible.
113)113)
Since there are no parameters of array type, this will only occur as the referenced type of a reference parameter.

12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]

1
#
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.
2
#
When comparing the basic forms of implicit conversion sequences (as defined in [over.best.ics])
3
#
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 initialized by L1 is less than the number of elements initialized by L2, or 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 a function lvalue and S2 binds an rvalue reference to a function lvalue
      [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 T1 can be converted to T2 by a qualification conversion.
      [Example 5: int f(const volatile int *); int f(const int *); int i; int j = f(&i); // calls f(const int*) — end example]
      or, if not that,
    • (3.2.6)
      S1 and S2 include reference bindings ([dcl.init.ref]), and the types to which the references refer are the same type except for top-level cv-qualifiers, and the type to which the reference initialized by S2 refers is more cv-qualified than the type to which the reference initialized by S1 refers.
      [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() } — 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 7: 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]
4
#
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 8: 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 9: 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]