键入trait以复制cv引用限定符

Question

在C++中编写类似库的代码我发现在copy_cv_reference_t类型特征中特别需要:

struct A;
struct B;

static_assert(std::is_same< copy_cv_reference_t<          A         , B >,          B          >{});
static_assert(std::is_same< copy_cv_reference_t<          A const   , B >,          B const    >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A         , B >, volatile B          >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A const   , B >, volatile B const    >{});
static_assert(std::is_same< copy_cv_reference_t<          A        &, B >,          B        & >{});
static_assert(std::is_same< copy_cv_reference_t<          A const  &, B >,          B const  & >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A        &, B >, volatile B        & >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A const  &, B >, volatile B const  & >{});
static_assert(std::is_same< copy_cv_reference_t<          A       &&, B >,          B       && >{});
static_assert(std::is_same< copy_cv_reference_t<          A const &&, B >,          B const && >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A       &&, B >, volatile B       && >{});
static_assert(std::is_same< copy_cv_reference_t< volatile A const &&, B >, volatile B const && >{});

我使用两种方法为自己创造它:通过类型限定符的id和仅通过SFINAE.

#include <type_traits>

#if 1
enum class type_qual_id
{
    value,
    const_value,
    lref,
    const_lref,
    rref,
    const_rref,
    volatile_value,
    volatile_const_value,
    volatile_lref,
    volatile_const_lref,
    volatile_rref,
    volatile_const_rref,
};

template< type_qual_id tqid, typename type > struct add_type_qualifier;
template< typename to > struct add_type_qualifier< type_qual_id::value               , to > { using type =          to         ; };
template< typename to > struct add_type_qualifier< type_qual_id::const_value         , to > { using type =          to const   ; };
template< typename to > struct add_type_qualifier< type_qual_id::lref                , to > { using type =          to       & ; };
template< typename to > struct add_type_qualifier< type_qual_id::const_lref          , to > { using type =          to const & ; };
template< typename to > struct add_type_qualifier< type_qual_id::rref                , to > { using type =          to       &&; };
template< typename to > struct add_type_qualifier< type_qual_id::const_rref          , to > { using type =          to const &&; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_value      , to > { using type = volatile to         ; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_const_value, to > { using type = volatile to const   ; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_lref       , to > { using type = volatile to       & ; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_const_lref , to > { using type = volatile to const & ; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_rref       , to > { using type = volatile to       &&; };
template< typename to > struct add_type_qualifier< type_qual_id::volatile_const_rref , to > { using type = volatile to const &&; };

template< type_qual_id tqid, typename to >
using add_qualifier_t = typename add_type_qualifier< tqid, to >::type;

template< typename type > constexpr type_qual_id get_type_qualifier_id                           = type_qual_id::value               ;
template< typename type > constexpr type_qual_id get_type_qualifier_id<          type const    > = type_qual_id::const_value         ;
template< typename type > constexpr type_qual_id get_type_qualifier_id<          type       &  > = type_qual_id::lref                ;
template< typename type > constexpr type_qual_id get_type_qualifier_id<          type const &  > = type_qual_id::const_lref          ;
template< typename type > constexpr type_qual_id get_type_qualifier_id<          type       && > = type_qual_id::rref                ;
template< typename type > constexpr type_qual_id get_type_qualifier_id<          type const && > = type_qual_id::const_rref          ;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type          > = type_qual_id::volatile_value      ;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type const    > = type_qual_id::volatile_const_value;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type       &  > = type_qual_id::volatile_lref       ;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type const &  > = type_qual_id::volatile_const_lref ;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type       && > = type_qual_id::volatile_rref       ;
template< typename type > constexpr type_qual_id get_type_qualifier_id< volatile type const && > = type_qual_id::volatile_const_rref ;

template< typename from, typename to >
using copy_cv_reference_t = add_qualifier_t< get_type_qualifier_id< from >, to >;

#else
#include <type_traits>

template< typename from, typename to >
struct copy_cv
{

    using type = to;

};

template< typename from, typename to >
struct copy_cv< from const, to >
    : copy_cv< from, to const >
{

};

template< typename from, typename to >
struct copy_cv< volatile from, to >
    : copy_cv< from, volatile to >
{

};

template< typename from, typename to >
struct copy_cv< volatile from const, to >
    : copy_cv< from, volatile to const >
{

};

template< typename from, typename to >
struct copy_reference 
{

    using type = to;

};

template< typename from, typename to >
struct copy_reference< from &, to >
    : copy_reference< from, to & >
{

};

template< typename from, typename to >
struct copy_reference< from &&, to >
    : copy_reference< from, to && >
{

};

template< typename from, typename to >
using copy_cv_reference_t = typename copy_reference< from, typename copy_cv< std::remove_reference_t< from >, to >::type >::type;

#endif

第一种方法看起来稍微有些人为,但提供了"类型限定符id"作为附加方,后者在某些情况下可能有用.第二种方法本质上是两步法.它可能有缺点.此外,它涉及std::remove_reference_t揭示cv限定类型.

一方面,我知道标准允许实现具有"内在"类型特征.另一方面,当代C++标准中目前没有类型特征.

copy_cv_reference_t类型特征的最佳实现是什么？不仅在两个之间.是否有更好的方法来实现它？有相应的提案吗？

命名怎么样？ids的顺序怎么样？

Answer 1

我没有遇到任何需要这种类型特征的用例,我不知道任何提议.因此,我只能提供更紧凑的实现,恕我直言更容易理解:

template<typename T,typename U>
struct copy_cv_reference
{
private:
    using R = std::remove_reference_t<T>;
    using U1 = std::conditional_t<std::is_const<R>::value, std::add_const_t<U>, U>;
    using U2 = std::conditional_t<std::is_volatile<R>::value, std::add_volatile_t<U1>, U1>;
    using U3 = std::conditional_t<std::is_lvalue_reference<T>::value, std::add_lvalue_reference_t<U2>, U2>;
    using U4 = std::conditional_t<std::is_rvalue_reference<T>::value, std::add_rvalue_reference_t<U3>, U3>;
public:
    using type = U4;
};

template<typename T,typename U>
using copy_cv_reference_t = typename copy_cv_reference<T,U>::type;

实例

你是否觉得它是一种改进是主观的.