c++ - 自己的 std::is_function 实现

Question

建议here以下列方式实现:

template<class Ret, class... Args>
struct is_function<Ret(Args...)const> : std::true_type {};
template<class Ret, class... Args>
struct is_function<Ret(Args...)volatile> : std::true_type {};

但它是一个有效的函数语法吗？ Visual Studio 2013 报错:

error C2270: 'abstract declarator' : modifiers not allowed on nonmember functions   

Answer 1

函数参数后的const或volatile称为cv-qualifier-seq。C++14 标准第 8.3.5 节第 6 段说:

A function type with a cv-qualifier-seq or a ref-qualifier (including a type named by typedef-name (7.1.3,14.1)) shall appear only as:

— the function type for a non-static member function,

— the function type to which a pointer to member refers,

— the top-level function type of a function typedef declaration or alias-declaration,

— the type-id in the default argument of a type-parameter (14.1), or

— the type-id of a template-argument for a type-parameter (14.3.1).

在您的示例中，Ret(Args...)const 和 Ret(Args...)volatile 满足最后一种情况。