[over.call.object]
12 Overloading [over]
12.2 Overload resolution [over.match]
12.2.2 Candidate functions and argument lists [over.match.funcs]
12.2.2.2 Function call syntax [over.match.call]
12.2.2.2.3 Call to object of class type [over.call.object]
If the postfix-expression E in 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 T are the results of a search for the name operator() in the scope of T.
In addition, for each non-explicit conversion function declared in T of the form
where the optional cv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes 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 name call-function and having a declaration of the form
is also considered as a candidate function.
[Note 1:
If a surrogate call function is selected by overload resolution, the behavior is as described in [over.call].
— end note]
Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration.97
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 2:
When comparing the call against the function call operators, the implied object argument is compared against the 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.
— end note]
[Example 1: 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]