c++ - Overload resolution without a parameter list on class member access? - 


struct x {     void f(double) {}     static void f(int) {} };  int main() {     x x;      auto y = x.f; }

gcc gives:

error: unable deduce ‘auto’ ‘x.x::f’

x.f class member access postfix-expression documented in 5.2.5 [expr.ref]

it says:

if f (possibly overloaded) member function, function overload resolution (13.3) used deter- mine whether x.f refers static or non-static member function. (from n3485 5.2.5.4.3 )

how can overload resolution applied here - x.f doesn't have parameter list overload resolution?

or missing something?

update: if change auto y = x.f line expression-statement:

- auto y = x.f; + x.f;

then gcc instead complains:

error: statement cannot resolve address of overloaded function

i think you're right in standard isn't precise @ point:

[expr.ref]/4

if f (possibly overloaded) member function, function overload resolution (13.3) used determine whether x.f refers static or non-static member function.

as first sub-bullet states:

if refers static member function , type of e2 “function of parameter-type-list returning t”, e1.e2 lvalue; expression designates static member function. type of e1.e2 same type of e2, namely “function of parameter-type-list returning t”.

that is, x.f can used initialize function ptr if f static, whereas (see following sub-bullet) if f not static, can used in function-call expression.

but actually, overload resolution is not performed select overload name of overloaded function outside context of function-call-expression:

[over.over]/1

a use of overloaded function name without arguments resolved in contexts function, pointer function or pointer member function specific function overload set. [...] the function selected 1 type identical function type of target type required in context.

that is, exact match, not invoking overload resolution mechanism described in [over.match]. therefore [expr.ref]/4 incomplete, doesn't describe case of using x.f initialize function pointer/reference.

as auto keyword doesn't specify target type (or according [dcl.spec.auto]/6, mechanism deduce type auto fails), cannot use auto y = x.f;:

  • x.f refers set of overloaded functions
  • a function set needs selected
  • selection based upon overload resolution (in function-call-expression, not applicable here) or based on target type
  • there's no target type here, or rather, target type equivalent template argument, therefore type deduction auto fails

the type of x.f therefore is

  • in context of function call, directly refer [expr.ref]/4
  • otherwise, set of overloaded functions, function selected according [over.over], whereupon type determined according [expr.ref]/4

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