Define ICSi(F) as
the implicit conversion sequence that converts
the ith argument in the list to the type of
the ith parameter of viable function F.
[over.best.ics] defines the implicit conversion sequences and [over.ics.rank]
defines what it means for one implicit conversion sequence to be
a better conversion sequence or worse conversion sequence than
another.
Given these definitions,
a viable function F1 is defined to be a
better
function than another viable function F2
if for all arguments i,
ICSi(F1) is not a worse conversion
sequence than ICSi(F2), and then
the context is an initialization by user-defined conversion
(see [dcl.init],
[over.match.conv], and [over.match.ref])
and the standard conversion sequence
from the result of F1 to the destination type
(i.e., the type of the entity being initialized)
is a better conversion sequence than the standard conversion sequence
from the result of F2 to the destination type
[Example 1: struct A {
A();
operatorint();
operatordouble();
} a;
int i = a; / a.operator int() followed by no conversion is better than/ a.operator double() followed by a conversion to intfloat x = a; / ambiguous: both possibilities require conversions,/ and neither is better than the other — end example]
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
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,
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 4: struct A {
A(int=0);
};
struct B: A {using A::A;
B();
};
int main(){
B b; / OK, B::B()} — end example]
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,
F1 is generated from a non-template constructor
and F2 is generated from a constructor template.
[Example 7: template<class T>struct A {using value_type = T;
A(value_type); / #1
A(const A&); / #2
A(T, T, int); / #3template<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 constructortemplate<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 initializetemplate<class T>
A(A<T>)-> A<A<T>>; / #7, as specialized as #5
A b2 = a; / uses #7 to deduce A<A<int>> and #1 to initialize — end example]
If there is exactly one viable function that is a better function
than all other viable functions, then it is the one selected by
overload resolution; otherwise the call is ill-formed.100
[Example 8: void Fcn(constint*, 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]
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.