[over.best.ics] (original) (raw)

12 Overloading [over]

12.2 Overload resolution [over.match]

12.2.4 Best viable function [over.match.best]

12.2.4.2 Implicit conversion sequences [over.best.ics]

12.2.4.2.1 General [over.best.ics.general]

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 1:

Other properties, such as the lifetime, storage duration, linkage, 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 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(); 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 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 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.

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_]

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.

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 a conversion to the explicit object parameter occurs, it can include user-defined conversion sequences.

— _end note_]

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 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); 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.

12.2.4.2.2 Standard conversion sequences [over.ics.scs]

Table 19summarizes 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_]

[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,PromotionorConversion,Qualification Adjustment.

— _end note_]

Each conversion in Table 19also 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.

12.2.4.2.3 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 ([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.

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.

12.2.4.2.4 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]).

12.2.4.2.5 Reference binding [over.ics.ref]

When a parameter of type “reference to cv T” binds directly ([dcl.init.ref]) to an argument expression:

[Example 1: struct A {};struct B : public A {} b;int f(A&);int f(B&);int i = f(b); void g() noexcept;int h(void (&)() noexcept); int h(void (&)()); int j = h(g); — _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 determined by the above rules.

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 of object type.

[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_]

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 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_]

12.2.4.2.6 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-listand the parameter is not a reference, 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); 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 array106and 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 2: void f(std::initializer_list<int>); f( {} ); f( {1,2,3} ); f( {'a','b'} ); f( {1.0} ); struct A { A(std::initializer_list<double>); A(std::initializer_list<std::complex<double>>); 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 3: struct A { A(std::initializer_list<int>);};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 ([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'} ); f( {1.0} ); — _end example_]

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'} ); 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.