16 Overloading [over]

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 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.

Otherwise, if the parameter type is a character array133 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.

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<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 ]

Otherwise, if the parameter type is “array of N X”, if there exists an implicit conversion sequence for each element of the array from the corresponding element of the initializer list (or from {} if there is no such element), 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 of X to perform the initialization of an object of type X from the argument initializer list:

• (6.1)

  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.

• (6.2)

  Otherwise, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence 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:

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 ]

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'} );        // OK: f(A(int,double)) user-defined conversion
f( {1.0} );             // error: narrowing

 — end example ]

Otherwise, if the parameter is a reference, see [over.ics.ref]. [ Note: The rules in this section 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'} );        // 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 ]

Otherwise, if the parameter type is not a class:

• (9.1)

  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:

  void f(int);
  f( {'a'} );             // OK: same conversion as char to int
  f( {1.0} );             // error: narrowing
  

   — end example ]

• (9.2)

  if the initializer list has no elements, the implicit conversion sequence is the identity conversion. [ Example:

  void f(int);
  f( { } );               // OK: identity conversion
  

   — end example ]

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