C 11 decltype成员

发布时间：2020-12-16 09:54:34 所属栏目：百科来源：网络整理

导读：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}; 但我能做到 class Foo { static void fn(); using fn_t = decltype(fn);}; This SO post声称： Within uneval

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
};

但我能做到

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

This SO post声称：

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

解决方法

fn是表示非静态成员函数的有效id-expression. §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表达式的少数上下文之一,因此该程序是不正确的.

（编辑：李大同）

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