[over.match] (original) (raw)

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

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.

So that argument and parameter lists are comparable within this heterogeneous set, a member function is considered to have an extra first parameter, called theimplicit object parameter, which represents the object for which the member function has been called.

For the purposes of overload resolution, both static and non-static member functions have an implicit object parameter, but constructors do not.

Similarly, when appropriate, the context can construct an argument list that contains animplied object argumentas the first argument in the list to denote the object to be operated on.

For non-static member functions, the type of the implicit object parameter is

whereXis the class of which the function is a member andcvis the cv-qualification on the member function declaration.

[ Example

:

For aconstmember function of classX, the extra parameter is assumed to have type “reference toconst X”.

— end example

]

For conversion 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 introduced by ausing-declarationinto 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

:

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

]

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 non-static 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

:

The fact that such an argument is an rvalue does not affect the ranking of implicit conversion sequences.

— end note

]

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

:

class T { public: T(); };

class C : T { public: C(int); }; T a = 1;

— end example

]

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-specifierwhose constant-expression is value-dependent ([temp.dep]), template argument deduction is performed first and then, if the context requires a candidate that is 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.117

A given name can refer to one or more function templates and also to 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 Dif the argument list has exactly one argument andC is reference-related to P andP is reference-related to D.

[ Example

:

struct A { A();
A(A &&);
template A(T &&);
}; struct B : A { using A::A; B(const B &);
B(B &&) = default;

struct X { X(X &&) = delete; } x; }; extern B b1; B b2 = static_cast<B&&>(b1);
struct C { operator B&&(); }; B b3 = C();

— end example

]

12.4.1.1 Function call syntax [over.match.call]

If the postfix-expression denotes an object of class type, overload resolution is applied as specified in [over.call.object].

If the postfix-expression is the address of an overload set, overload resolution is applied using that set as described above.

If the function selected by overload resolution is a non-static member function, the program is ill-formed.

[ Note

:

The resolution of the address of an overload set in other contexts is described in [over.over].

— end note

]

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

In qualified function calls, the name to be resolved is anid-expressionand is preceded by an->or.operator.

Since the constructA->Bis 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 thepostfix-expressionthat is the left operand of the.operator has type “cv T” whereTdenotes a class.118

Under this assumption, theid-expressionin the call is looked up as a member function ofTfollowing the rules for looking up names in classes ([class.member.lookup]).

The function declarations found by that lookup constitute the set of candidate functions.

The argument list is theexpression-listin 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]).

In unqualified function calls, the name is not qualified by an->or.operator and has the more general form of aprimary-expression.

The name is looked up in the context of the function call following the normal rules for name lookup in expressions ([basic.lookup]).

The function declarations found by that lookup constitute the set of candidate functions.

Because of the rules for name lookup, the set of candidate functions consists (1) entirely of non-member functions or (2) entirely of member functions of some classT.

In case (1), the argument list is the same as theexpression-listin the call.

In case (2), the argument list is theexpression-listin the call augmented by the addition of an implied object argument as in a qualified function call.

If the keywordthis is in scope and refers to classT, or a derived class ofT, then the implied object argument is(*this).

If the keywordthisis not in scope or refers to another class, then a contrived object of typeTbecomes the implied object argument.119

If the argument list is augmented by a contrived object and overload resolution selects one of the non-static member functions ofT, the call is ill-formed.

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

If the postfix-expression Ein 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 Tare obtained by ordinary lookup of the name operator()in the context of (E).operator().

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 optionalcv-qualifier-seqis the same cv-qualification as, or a greater cv-qualification than,cv, and whereconversion-type-iddenotes 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 namecall-functionand 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 ofTprovided the function is not hidden withinTby another intervening declaration.120

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

:

When comparing the call against the function call operators, the implied object argument is compared against the implicit 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.

The conversion function from which the surrogate call function was derived will be used in the conversion sequence for that parameter since it converts the implied object argument to the appropriate function pointer or reference required by that first parameter.

— end note

]

[ Example

:

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);

— end example

]

12.4.1.2 Operators in expressions [over.match.oper]

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

:

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

:

struct String { String (const String&); String (const char*); operator const char* (); }; String operator + (const String&, const String&);

void f() { const char* p= "one" + "two";

int I = 1 + 1;

}

— end example

]

If either operand has a type that is a class or an enumeration, a user-defined operator function might 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 15(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 15: Relationship between operator and function call notation [tab:over.match.oper]

Subclause Expression As member function As non-member function
[over.unary] @a (a).operator@ ( ) operator@(a)
[over.binary] a@b (a).operator@ (b) operator@(a, b)
[over.ass] a=b (a).operator= (b)
[over.sub] a[b] (a).operator[](b)
[over.ref] a-> (a).operator->( )
[over.inc] a@ (a).operator@ (0) operator@(a, 0)

For a unary operator @with an operand of type cv1 T1, and for a binary operator @with a left operand of type cv1 T1and a right operand of type cv2 T2, four sets of candidate functions, designatedmember candidates,non-member candidates,built-in candidates, andrewritten candidates, are constructed as follows:

For the built-in assignment operators, conversions of the left operand are restricted as follows:

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].121

[ Example

:

struct A { operator int(); }; A operator+(const A&, const A&); void m() { A a, b; a + b;
}

— end example

]

If a rewritten operator<=> candidate is selected by overload resolution for an operator @,x @ yis interpreted as0 @ (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, andx @ y is interpreted as:

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].

[ Example

:

struct X { operator double(); };

struct Y { operator int*(); };

int *a = Y() + 100.0;
int *b = Y() + X();

— end example

]

The second operand of operator->is ignored in selecting anoperator->function, and is not an argument when theoperator->function is called.

Whenoperator->returns, the operator->is applied to the value returned, with the original second operand.122

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].

[ Note

:

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);
a + a;
}

— end note

]

12.4.1.3 Initialization by constructor [over.match.ctor]

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 that is not in the context of copy-initialization, the candidate functions are all the constructors of the class of the object being initialized.

For copy-initialization (including default initialization in the context of copy-initialization), the candidate functions are all the converting constructors ([class.conv.ctor]) of that class.

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

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

:

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, withTa class type, the candidate functions are selected as follows:

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

[ Note

:

This argument will be compared against the first parameter of the constructors and against the implicit object parameter of the conversion functions.

— end note

]

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

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 “cv1 T” is the type of the object being initialized, and “cv S” is the type of the initializer expression, withSa class type, the candidate functions are selected as follows:

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

[ Note

:

This argument will be compared against the implicit object parameter of the conversion functions.

— end note

]

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

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, and “cv S” is the type of the initializer expression, withSa class type, the candidate functions are selected as follows:

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

[ Note

:

This argument will be compared against the implicit object parameter of the conversion functions.

— end note

]

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

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:

In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.

[ Note

:

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

]

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

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.

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

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

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 .

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-idof the defining-type-id, the template arguments of the return type of fare deduced from the defining-type-id of Aaccording to the process in [temp.deduct.type]with the exception that deduction does not fail if not all template arguments are deduced.

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:

The arguments of a template A are said to be deducible from a type T if, given a class template

template class AA;

with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of Awith 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:

All such notional constructors are considered to be public members of the hypothetical class type.

[ Example

:

template struct A { explicit A(const T&, ...) noexcept;
A(T&&, ...);
};

int i; A a1 = { i, i };

A a2{i, i};
A a3{0, i};
A a4 = {0, i};

template A(const T&, const T&) -> A<T&>;
template explicit A(T&&, T&&) -> A;

A a5 = {0, 1};
A a6{0,1};
A a7 = {0, i};
A a8{0,i};

template struct B { template using TA = T; template B(U, TA); };

B b{(int*)0, (char*)0};

template struct S { T x; T y; };

template struct C { S s; T t; };

template struct D { S s; T t; };

C c1 = {1, 2};
C c2 = {1, 2, 3};
C c3 = {{1u, 2u}, 3};

D d1 = {1, 2};
D d2 = {1, 2, 3};

template struct E { T t; decltype(t) t2; };

E e1 = {1, 2};

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>{}, {}, {}};
F f2 = {Types<X, Y, Z>{}, X{}, Y{}};
F f3 = {Types<X, Y, Z>{}, X{}, W{}};

— end example

]

[ Example

:

template <class T, class U> struct C { C(T, U);
}; template<class T, class U> C(T, U) -> C<T, std::type_identity_t>;

template using A = C<V *, V *>; template<std::integral W> using B = A;

int i{}; double d{}; A a1(&i, &i);
A a2(i, i);
A a3(&i, &d);

B b1(&i, &i);
B b2(&d, &d);

Possible exposition-only implementation of the above procedure:

template class AA; template class AA<A> { }; template concept deduces_A = requires { sizeof(AA); };

template<T, U> auto f1(T, U) -> C<T, U>;

template requires deduces_A<C<V *, V *>> auto f1_prime(V , V) -> C<V *, V *>;

template<class T, class U> auto f2(T, U) -> C<T, std::type_identity_t>;

template<class V, class U> requires deduces_A<C<V *, std::type_identity_t>> auto f2_prime(V *, U) -> C<V *, std::type_identity_t>;

template class BB; template class BB<B> { }; template concept deduces_B = requires { sizeof(BB); };

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>> && deduces_B<C<W *, std::type_identity_t>> auto f2_prime_for_B(W *, U) -> C<W *, std::type_identity_t>;

— end example

]

12.4.2 Viable functions [over.match.viable]

From the set of candidate functions constructed for a given context ([over.match.funcs]), a set of viable functions is chosen, from which the best function will be selected by comparing argument conversion sequences and associated constraints ([temp.constr.decl]) for the best fit ([over.match.best]).

The selection of viable functions considers associated constraints, if any, and relationships between arguments and function parameters other than the ranking of conversion sequences.

First, to be a viable function, a candidate function shall have enough parameters to agree in number with the arguments in the list.

Third, forFto be a viable function, there shall exist for each argument animplicit conversion sequence that converts that argument to the corresponding parameter ofF.

If the parameter has reference type, the implicit conversion sequence includes the operation of binding the reference, and the fact that an lvalue reference to non-const cannot be bound to an rvalue and that an rvalue reference cannot be bound to an lvalue can affect the viability of the function (see [over.ics.ref]).

12.4.3 Best viable function [over.match.best]

Define ICSi(F) as follows:

Given these definitions, a viable functionF1is defined to be abetterfunction than another viable functionF2if for all argumentsi, ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then

float x = a;

— end example
]
or, if not that,

};
A x(1, 2, 3);
template
A(T) -> A;
A a(42);
A b = a;
template
A(A) -> A<A>;
A b2 = a;
— 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.124

[ Example

:

void Fcn(const int*, short); void Fcn(int*, int);

int i; short s = 0;

void f() { Fcn(&i, s);

Fcn(&i, 1L);

Fcn(&i, 'c');

}

— end example

]

If the best viable function resolves to a function for which multiple declarations were found, and if at least two of these declarations — or the declarations they refer to in the case ofusing-declarations— specify a default argument that made the function viable, the program is ill-formed.

[ Example

:

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);
f();
}

— end example

]

12.4.3.1 Implicit conversion sequences [over.best.ics]

An implicit conversion sequenceis 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]).

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

:

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

]

A well-formed implicit conversion sequence is one of the following forms:

However, if the target is

and the constructor or user-defined conversion function is a candidate by

user-defined conversion sequences are not considered.

[ Note

:

These rules prevent more than one user-defined conversion from being applied during overload resolution, thereby avoiding infinite recursion.

— end note

]

[ Example

:

struct Y { Y(int); }; struct A { operator int(); }; Y y1 = A();

struct X { X(); }; struct B { operator X(); }; B b; X x{{b}};

— end example

]

For the case where the parameter type is a reference, see [over.ics.ref].

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

:

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

:

A parameter of typeAcan be initialized from an argument of typeconst A.

The implicit conversion sequence for that case is the identity sequence; it contains no “conversion” fromconst AtoA.

— end example

]

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-baseconversion from the derived class to the base class.

[ Note

:

There is no such standard conversion; this derived-to-base conversion exists only in the description of implicit conversion sequences.

— end note

]

A derived-to-base conversion has Conversion rank ([over.ics.scs]).

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.

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]).

If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.

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 theambiguous 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

:

This rule prevents a function from becoming non-viable because of an ambiguous conversion sequence for one of its parameters.

[ Example

:

class B; class A { A (B&);}; class B { operator A (); }; class C { C (B&); }; void f(A) { } void f(C) { } B b; f(b);

void f(B) { } f(b);

— 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.

The three forms of implicit conversion sequences mentioned above are defined in the following subclauses.

12.4.3.1.1 Standard conversion sequences [over.ics.scs]

Table 16summarizes the conversions defined in [conv] and partitions them into four disjoint categories: Lvalue Transformation, Qualification Adjustment, Promotion, and Conversion.

[ Note

:

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

]

[ Note

:

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,PromotionorConversion,Qualification Adjustment.

— end note

]

Each conversion in Table 16also has an associated rank (Exact Match, Promotion, or Conversion).

These are used to rank standard conversion sequences.

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.

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

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, the initial standard conversion sequence converts the source type to the type required by the argument of the constructor.

If the user-defined conversion is specified by a conversion function, the initial standard conversion sequence converts the source type to the implicit object parameter of the conversion function.

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]).

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.

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.

12.4.3.1.3 Ellipsis conversion sequences [over.ics.ellipsis]

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]).

12.4.3.1.4 Reference binding [over.ics.ref]

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

:

struct A {}; struct B : public A {} b; int f(A&); int f(B&); int i = f(b);

— 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, with the second standard conversion sequence 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.

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.

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

:

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

]

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

:

A function with an “lvalue reference to int” parameter can be a viable candidate even if the corresponding argument is anintbit-field.

The formation of implicit conversion sequences treats theintbit-field as anintlvalue 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

]

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

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.

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

:

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

:

struct A { int x, y; }; struct B { int y, x; }; void f(A a, int);
void f(B b, ...);
void g(A a);
void g(B b);
void h() { f({.x = 1, .y = 2}, 0);
f({.y = 2, .x = 1}, 0);

g({.x = 1, .y = 2});
}

— end example

]

— end note

]

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 Xor a class derived from X, the implicit conversion sequence is the one required to convert the element to the parameter type.

Otherwise, if the parameter type is a character array125and the initializer list has a single element that is an appropriately-typedstring-literal ([dcl.init.string]), the implicit conversion sequence is the identity conversion.

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

:

void f(std::initializer_list); f( {} );
f( {1,2,3} );
f( {'a','b'} );
f( {1.0} );

struct A { A(std::initializer_list);
A(std::initializer_list<complex>);
A(std::initializer_liststd::string);
}; A a{ 1.0,2.0 };

void g(A); g({ "foo", "bar" });

typedef int IA[3]; void h(const IA&); h({ 1, 2, 3 });

— end example

]

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.

Otherwise, if the parameter is a non-aggregate class X and overload resolution per [over.match.list] chooses a single best constructor C ofX to perform the initialization of an object of type X from the argument initializer list:

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

:

struct A { A(std::initializer_list); }; void f(A); f( {'a', 'b'} );

struct B { B(int, double); }; void g(B); g( {'a', 'b'} );
g( {1.0, 1.0} );

void f(B); f( {'a', 'b'} );

struct C { C(std::string); }; void h(C); h({"foo"});

struct D { D(A, C); }; void i(D); i({ {1,2}, {"bar"} });

— end example

]

Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion.

[ Example

:

struct A { int m1; double m2; };

void f(A); f( {'a', 'b'} );
f( {1.0} );

— end example

]

Otherwise, if the parameter is a reference, see [over.ics.ref].

[ Note

:

The rules in this subclause will apply for initializing the underlying temporary for the reference.

— end note

]

[ Example

:

struct A { int m1; double m2; };

void f(const A&); f( {'a', 'b'} );
f( {1.0} );

void g(const double &); g({1});

— end example

]

Otherwise, if the parameter type is not a class:

In all cases other than those enumerated above, no conversion is possible.

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

This subclause defines a partial ordering of implicit conversion sequences based on the relationshipsbetter conversion sequenceandbetter 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 aworse conversion sequencethan S1.

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

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

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

— end example
]

— 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: