成员的 C++11 decltype

Question

为什么我不能这样做:

class Foo {
  void fn();
  using fn_t = decltype(fn); //call to non-static member function without an object argument
};

但我可以

class Foo {
  static void fn();
  using fn_t = decltype(fn);
};

This SO post声明:

Within unevaluated operands (operands of decltype, sizeof, noexcept, ...) you can name nonstatic data members also outside of member functions

Answer 1

fn 是一个有效的 id-expression 表示一个非静态成员函数。 §5.1.1 [expr.prim.general]/p13(省略脚注):

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

• as part of a class member access (5.2.5) in which the object expression refers to the member’s class or a class derived from that class, or
• to form a pointer to member (5.3.1), or
• if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

§7.1.6.2 [dcl.type.simple]/p4:

The operand of the decltype specifier is an unevaluated operand (Clause 5).

由于 decltype 不是可以使用表示非静态成员函数的 id-expression 的少数上下文之一，因此程序格式错误。