is_member_function_pointer implementation

is_member_function_pointer implementation

本文关键字：implementation pointer member is function      更新时间：2023-10-16

我正在尝试实现我自己的is_member_function_pointer，但遇到了问题。

namespace __implementation
{
    // integral_constant
    template<typename T, T v>
    struct integral_constant
    {
        static constexpr T result = v;
        typedef integral_constant<T,v> type;
        constexpr operator T() { return result; }
    };
    // true_type
    typedef integral_constant<bool, true> true_type;
    // false_type
    typedef integral_constant<bool, false> false_type;
    // remove_const
    template<typename T> struct remove_const { typedef T type; };
    template<typename T> struct remove_const<const T> { typedef T type; };
    // remove_volatile
    template<typename T> struct remove_volatile { typedef T type; };
    template<typename T> struct remove_volatile<volatile T> { typedef T type; };
    // remove_cv
    template<typename T> struct remove_cv
    { typedef typename remove_volatile<typename remove_const<T>::type>::type type; };
    // is_function - ugliness ahead due to variadic functions!
    template<typename T, typename... Args> struct is_function : public false_type {};
    template<typename T, typename... Args> struct is_function<T(Args...)> : public true_type {}; // normal function
    template<typename T, typename... Args> struct is_function<T(Args......)> : public true_type {}; // variadic function
    // is_member_function_pointer
    template<typename T> struct is_member_function_pointer : public false_type {};
    template<typename T, typename Class> struct is_member_function_pointer<T Class::*> : public is_function<T> {};   
}
/*
 * Short forms: either alias templates or constexpr functions
 */
// remove_const
template<typename T>
using remove_const = typename __implementation::remove_const<T>::type;
// remove_volatile
template<typename T>
using remove_volatile = typename __implementation::remove_volatile<T>::type;
// remove_cv
template<typename T>
using remove_cv = typename __implementation::remove_cv<T>::type;
// is_member_function_pointer
template<typename T>
constexpr bool is_member_function_pointer { return __implementation::is_member_function_pointer<T>::result; }
// is_function
template<typename T>
constexpr bool is_function() { return __implementation::is_function<typename __implementation::remove_cv<T>::type>::result; }

问题在于普通的函数指针，它们不能像member_function_pointer模板那样正常工作。此外，const成员函数指针不能被识别为member_function_pointer，这是很不幸的。如何修复此实现？我相信我可以enable_if一个常规函数指针专门化来解决普通函数指针问题，但我看不到解决const成员函数指针问题的方法（我确实尝试过在类型特征定义中到处添加remove_cv和const，但都无济于事）。我可以看到当前的is_member_function_pointer不能处理const类的成员函数，但我不知道我能用什么语法魔法来做到这一点

感谢您的帮助。

在Chat上的一次简短谈话带来了这个简单的解决方案：

// is_member_function_pointer
template<typename T> struct is_member_function_pointer : public false_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...)> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) volatile> : public true_type {};  
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const volatile> : public true_type {};

其中还包括volatile的专业化。

编辑正如@Xeo和@Johannes所指出的，我错过了ref限定符（也就是*this的rvalue）版本：

template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) volatile &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const volatile &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) volatile &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...) const volatile &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...)> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) volatile> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const volatile> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) volatile &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const volatile &> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) volatile &&> : public true_type {};
template<typename T, typename Class, typename... Args> struct is_member_function_pointer<T (Class::*)(Args...,...) const volatile &&> : public true_type {};

这仍然是可行的。它比疯狂的模板魔术和众多的助手模板IMHO更清晰。

EDIT2要澄清的是，以上内容都在一个实现命名空间中，并由封装

template<typename T> constexpr bool
is_member_function_pointer() { return __implementation::is_member_function_pointer<remove_cv<T>>::result; }

其中remove_cv是方便的别名模板

template<typenamen T> using
remove_cv = typename __implementation::remove_cv<T>::type;

我想还有其他更好的方法，但这一方法至少对读者来说是清楚的，没有任何进一步的SFINAE技巧，比如在libc++或libstdc++中应用，IMHO。

或者

template<typename T> struct remove_c { typedef T type; };
template<typename T> struct remove_c<T const> { typedef T type; };
template<bool C> 
struct bool_ { static bool const value = C; };
template<typename T, typename U>
struct is_same : bool_<false> {};
template<typename T>
struct is_same<T, T> : bool_<true> { };
template<typename T, typename = bool_<true> >
struct is_function : bool_<false> {};
struct C { };
template<typename T>
struct is_function< T, 
   bool_<is_same<typename remove_c<T>::type const C::*, 
                 typename remove_c<T>::type       C::*>::value> > 
  : bool_<true> {};
template<typename T>
struct is_member_function_pointer_impl : bool_<false> {};
template<typename T, typename C>
struct is_member_function_pointer_impl<T C::* const volatile>
  : is_function<T> {};
template<typename T>
struct is_member_function_pointer 
  : is_member_function_pointer_impl<T const volatile> {};
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