C ++ 11 decltype member
Why can't I do this:
class Foo {
void fn();
using fn_t = decltype(fn); //call to non-static member function without an object argument
};
But I can do
class Foo {
static void fn();
using fn_t = decltype(fn);
};
This SO post states:
Inside non-operable operands (operands decltype, sizeof, noexcept, ...) you can name non-static data members also outside of member functions
fn
is a valid id expression that denotes a non-stationary member function. §5.1.1 [expr.prim.general] / p13 (footnote omitted):
An identifier that denotes a non-static data member or non-static class member function can only be used:
- as part of a member access of a class (5.2.5) in which an object expression refers to a member class or a class derived from that class, or
- to form a pointer to an element (5.3.1) or
- if the id expression denotes a non-static data item and it appears in an unvalued operand.
§7.1.6.2 [dcl.type.simple] / p4:
The operand of the qualifier
decltype
is an unvalued operand (Item 5).
Since it is decltype
not one of the few contexts in which id can be used to denote a non-stationary member function, the program is ill-formed.