用于复制 cv 引用限定符的类型特征

Question

Writing library-like code in C++ I found there is particular need in copy_cv_reference_t type trait:

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 && >{});

I invent it for myself using two approaches: via means of id of type qualifiers and via SFINAE only.

#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

First approach looks slightly more artificial, but provides a "type qualifiers id" as additional side and latter can be useful in some situations. Second approach is inherently two-step one. It could has downsides. In addition, it involve std::remove_reference_t to reveal the cv-qualified type.

On the one hand, I know standard allows for implementations to have an "intrinsic" type traits . On the other hand, there is no the type trait currently in contemporary C++ standard.

What is the best implementation of copy_cv_reference_t type trait? Not only between above two. Are there better approaches to implement it? Is there corresponding proposal?

What about naming? What about order of ids?

Answer 1

I haven't encountered any use-case which required a type-trait like that and I am not aware of any proposal. I can therefore only offer an implementation which is more compact and IMHO more simple to understand:

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;

Live example

Whether you find it an improvement is subjective.

Answer 2

Here is a boost::hana esque system for qualifiers, not references.

enum class qualifier:unsigned char {
  none,
  is_const = 1<<1,
  is_volatile = 1<<2,
};
constexpr inline qualifier operator|(qualifier lhs,qualifier rhs){
  return qualifier( unsigned(lhs)|unsigned(rhs) );
}
constexpr inline bool operator&(qualifier lhs,qualifier rhs){
  return unsigned(lhs)&unsigned(rhs);
}
// not a simple alias to make operator overloading work right:
template<qualifier q>
struct qual_t:std::integral_constant<qualifier,q> {};
template<qualifier lhs, qualifier rhs>
constexpr qual_t<lhs|rhs> operator|(qual_t<lhs>,qual_t<rhs>){return {};}


template<class T>struct tag{using type=T;};
template<class Tag>using type_t=typename Tag::type;

template<class T>
constexpr qual_t<
  (std::is_const<T>{}?qualifier::is_const:qualifier::none)
 |(std::is_volatile<T>{}?qualifier::is_volatile:qualifier::none)
> qual(tag<T>={}){ return {}; }

template<class B, qualifier q,
  class Step1=std::conditional_t<q&qualifier::is_const,const B,B>,
  class R=std::conditional_t<q&qualifier::is_volatile,volatile Step1, Step1>
>
constexpr tag<R> add_qual( tag<B>={}, qual_t<q>={} ){ return {}; }

template<class T,qualifier Q>
auto operator+( tag<T> t, qual_t<Q> q ){
  return add_qual(t,q);
}

template<class B, qualifier q>
using add_qual_t=type_t<decltype(tag<B>{}+qual_t<q>{})>;

using the above you can work with tag<T> types or raw types.

Divorcing reference work from qualifier makes sense to me.

Want to see copy?

template<class From, class To>
constexpr auto copy_qual(tag<From> from={}, tag<To> to={}){
  return to + qual(from);
}

which can be converted to types:

template<class From, class To>
using copy_qual_t=type_t<decltype(copy_qual<From,To>())>;

with a bit more ugly.

We can do the exact same thing with references

enum class ref_qualifier:unsigned char {
  none,
  rvalue,
  lvalue
};

including reference collapsing

constexpr inline ref_qualfier operator|(ref_qualifier lhs, ref_qualifier rhs){
  return ((unsigned)lhs>(unsigned)rhs)?lhs:rhs;
}
constexpr inline ref_qualfier operator&(ref_qualifier lhs, ref_qualifier rhs){
  return ((unsigned)lhs>(unsigned)rhs)?rhs:lhs;
}

etc. (both lvalue and rvalue qualifiers ends with lvalue)

We can write add_ref_qual and sub_ref_qual , overload + and - with tag s.

template<class From, class To>
constexpr auto copy_ref_and_quals( tag<From> from, tag<To> to ) {
  auto from_ref = ref_qual(from);
  auto from_cv = qual(from-from_ref);
  auto to_ref = ref_qual(to);
  return (to-to_ref)+from_cv+to_ref+from_ref;
}

where we strip the ref qualification off to, then add the cv qualification of from, then add back in the ref qualifiactions of both from and to.

Answer 3

I advise you to decompose your trait/metafunction in two. First of all, it's good separation of concerns: the two tasks of propagating cv-qualifiers and propagating ref-qualifiers really are different. I sometimes use of the two in isolation, too. Eg with pointers qualifying_cv_of_t<A, B>* comes up from time to time, in which case we absolutely don't want pointers to references as those are invalid. (My traits are named qualifying_*_of_t<A, B> which can be understood to mean 'the relevant properties of A are qualifying those of B '.)

Second, the latter trait is rather tricky to get right. Maybe you want to mechanically copy the top-level reference (if present), in which case there isn't much to say about it. On the other hand, you say:

[…] some kind of unwrapping (say, for variant, optional, tuple etc.) […]

which is definitively one of the scenarios where I use it. One of the thing I've decided is that it's not actually references that I care about, it's value category. That is to say, qualifying_t<X, Y> (the one that propagates everything) conceptually represents decltype(expr.member) † where expr has type

struct X { Y member; };

possibly cv-qualified. The trait makes it possible to write eg

template<typename T> qualifying_t<T, U> foo(T&& t)
{ return std::forward<T>(t).u; }

correctly (assuming u does have type U ), even if eg U is a reference type. So, how tricky is that? Well, even the Standard Committee has yet to figure out all the details for the C++14 → C++1z transition to fix a bug introduced in the C++11 → C++14 transition. I won't spell out a solution because I don't believe one size fits all: eg std::tuple_element_t and std::get form a very similar trait/function template pair that does something different than what I outlined above.

The good thing is that you can write as many trait as you need, combine it with your qualifying_cv_of and you are good to go (and I fact I have two such traits myself!). So maybe the real answer is not to split the trait in two, but in however many you need.

---

†: the keen-eyed may have noticed something off here and would have instead assumed something like decltype( (expr.member) ) . I do not have a satisfactory answer yet as to which is preferable, or why.

Answer 4

Here is a plug and play solution to your problem:

#include<type_traits>
#include<cstddef>

static const std::size_t N = 42;

template<std::size_t N>
struct choice: choice<N-1> {};

template<>
struct choice<0> {};

template<typename T, typename U>
struct types {
    using basic = T;
    using decorated = U;
};

template<typename T, typename U>
auto f(choice<0>) { return types<T, U>{}; }

template<typename T, typename U, typename = std::enable_if_t<std::is_lvalue_reference<T>::value>>
auto f(choice<1>) {
    auto t = f<std::remove_reference_t<T>, U>(choice<N>{});
    using B = typename decltype(t)::basic;
    using D = typename decltype(t)::decorated;
    return types<B, std::add_lvalue_reference_t<D>>{};
}

template<typename T, typename U, typename = std::enable_if_t<std::is_rvalue_reference<T>::value>>
auto f(choice<2>) {
    auto t = f<std::remove_reference_t<T>, U>(choice<N>{});
    using B = typename decltype(t)::basic;
    using D = typename decltype(t)::decorated;
    return types<B, std::add_rvalue_reference_t<D>>{};
}

template<typename T, typename U, typename = std::enable_if_t<std::is_const<T>::value>>
auto f(choice<3>) {
    auto t = f<std::remove_const_t<T>, U>(choice<N>{});
    using B = typename decltype(t)::basic;
    using D = typename decltype(t)::decorated;
    return types<B, std::add_const_t<D>>{};
}

template<typename T, typename U, typename = std::enable_if_t<std::is_volatile<T>::value>>
auto f(choice<4>) {
    auto t = f<std::remove_volatile_t<T>, U>(choice<N>{});
    using B = typename decltype(t)::basic;
    using D = typename decltype(t)::decorated;
    return types<B, std::add_volatile_t<D>>{};
}

template<typename T, typename U>
auto f() {
    return f<T, U>(choice<N>{});
}

template<typename T, typename U = char>
using copy_cv_reference_t = typename decltype(f<T, U>())::decorated;

struct A;
struct B;

int main() {
    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 && >{}, "!");
}

Answer 5

A concise way to implement this is with a little helper utility that applies a metafunction based on some condition:

template <template <typename...> class MFn, bool condition, typename T>
using apply_if_t = std::conditional_t<condition, MFn<T>, T>;

This allows us to compose the different cvref qualifiers:

template <typename T>
using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;

template <typename From, typename To>
using copy_cv_t =
    apply_if_t<std::add_volatile_t, std::is_volatile_v<From>,
        apply_if_t<std::add_const_t, std::is_const_v<From>,
            std::remove_cv_t<To>>>;

template <typename From, typename To>
using copy_ref_t =
    apply_if_t<std::add_rvalue_reference_t, std::is_rvalue_reference_t<From>,
        apply_if_t<std::add_lvalue_reference_t, std::is_lvalue_reference_t<From>,
            std::remove_reference_t<To>>>;

template <typename From, typename To>
using copy_cvref_t = copy_ref_t<From,
    copy_cv_t<std::remove_reference_t<From>, remove_cvref_t<To>>>;