c++ - Stripping all qualifiers from a function type

Question

Given a possibly varargs function type with possibly a cv-qualifier-seq and possibly a ref-qualifier, is it possible to write a type trait that strips all the qualifiers without writing 4 * 3 * 2 = 24 partial specializations?

template<class T>
struct strip_function_qualifiers;

template<class R, class... Args>
struct strip_function_qualifiers<R(Args...)> { using type = R(Args...); };

template<class R, class... Args>
struct strip_function_qualifiers<R(Args..., ...)> { using type = R(Args..., ...); };

template<class R, class... Args>
struct strip_function_qualifiers<R(Args...) const> { using type = R(Args...); };

template<class R, class... Args>
struct strip_function_qualifiers<R(Args..., ...) const > { using type = R(Args..., ...); };

template<class R, class... Args>
struct strip_function_qualifiers<R(Args...) const &> { using type = R(Args...); };

template<class R, class... Args>
struct strip_function_qualifiers<R(Args..., ...) const & > { using type = R(Args..., ...); };

// etc. etc. for each possible combination (24 in total)

And with the new transactional memory TS adding transaction_safe to the mix, does that mean we'll need to write 48 partial specializations for this?

---

Edit: Quoting the description of these weird function types ([dcl.fct]/p6, quoting N4140):

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).

[ Example:

typedef int FIC(int) const;
FIC f; // ill-formed: does not declare a member function
struct S {
   FIC f; // OK
};
FIC S::*pm = &S::f; // OK

— end example ]

The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type. In the latter case, the cv-qualifiers are ignored. [Note: a function type that has a cv-qualifier-seq is not a cv-qualified type; there are no cv-qualified function types. — end note ] [ Example:

 typedef void F();
 struct S {
    const F f; // OK: equivalent to: void f();
 };

— end example ]

The return type, the parameter-type-list, the ref-qualifier, and the cv-qualifier-seq, but not the default arguments (8.3.6) or the exception specification (15.4), are part of the function type. [Note: Function types are checked during the assignments and initializations of pointers to functions, references to functions, and pointers to member functions. — end note ]

Answer 1

I see no way around this - define it once and reuse it whenever possible.
Such a huge amount of specializations is avoidable when the qualifiers are top-level - we could use std::remove_cv or std::remove_reference in that case, removing all orthogonal qualifiers in each step. Unfortunately this is not applicable for functions as explained in the paragraph quoted by you: The e.g. cv-qualifier is part of the function type, and not top-level. void() const is a fundamentally different type than void(), and thus the both have to be matched by two different partial specializations.

You can shorten all specializations using macros though:

#define REM_CTOR(...) __VA_ARGS__

#define SPEC(var, cv, ref) 
template <typename R, typename... Args> 
struct strip_function_qualifiers<R(Args... REM_CTOR var) cv ref > 
{using type = R(Args... REM_CTOR var);};

#define REF(var, cv) SPEC(var, cv,) SPEC(var, cv, &) SPEC(var, cv, &&)

#define CV(var) REF(var,) REF(var, const) 
                REF(var, volatile) REF(var, const volatile)

template <typename> struct strip_function_qualifiers;

CV(()) CV((,...))

Demo.
Boost.PP is possible as well:

#include <boost/preprocessor/tuple/enum.hpp>
#include <boost/preprocessor/seq/elem.hpp>
#include <boost/preprocessor/seq/for_each_product.hpp>

#define REF  (&&)(&)()
#define CV   (const volatile)(const)(volatile)()
#define VAR  (())((,...)) // Had to add a comma here and use rem_ctor below,
                          // otherwise Clang complains about ambiguous ellipses

#define SPEC(r, product) 
template <typename R, typename... Args> 
struct strip_function_qualifiers<R(Args... BOOST_PP_TUPLE_ENUM( 
    BOOST_PP_SEQ_ELEM(0, product))) 
    BOOST_PP_SEQ_ELEM(1, product)   
    BOOST_PP_SEQ_ELEM(2, product)>  
{using type = R(Args... BOOST_PP_TUPLE_ENUM(BOOST_PP_SEQ_ELEM(0, product)));};

template <typename> struct strip_function_qualifiers;

BOOST_PP_SEQ_FOR_EACH_PRODUCT(SPEC, (VAR)(CV)(REF))

Demo. Both methods won't get much longer when adding new qualifiers such as transaction_safe or transaction_safe_noinherit.

---

Here is a modified SPEC that also defines certain trait members.

#include <type_traits>

#include <boost/preprocessor/tuple/size.hpp>

// […]

#define SPEC(r, product)                                         
template <typename R, typename... Args>                          
struct strip_function_qualifiers<R(Args... BOOST_PP_TUPLE_ENUM(  
    BOOST_PP_SEQ_ELEM(0, product))) 
    BOOST_PP_SEQ_ELEM(1, product)   
    BOOST_PP_SEQ_ELEM(2, product)>  
{                                     
    using type = R(Args... BOOST_PP_TUPLE_ENUM(BOOST_PP_SEQ_ELEM(0, product))); 
                                                            
private:                                                    
    using cv_type = int BOOST_PP_SEQ_ELEM(1, product);      
    using ref_type = int BOOST_PP_SEQ_ELEM(2, product);     
public:                                                     
    using is_const    = std::is_const<cv_type>;             
    using is_volatile = std::is_volatile<cv_type>;          
    using is_ref_qualified = std::is_reference<ref_type>;               
    using is_lvalue_ref_qualified = std::is_lvalue_reference<ref_type>; 
    using is_rvalue_ref_qualified = std::is_rvalue_reference<ref_type>; 
    using is_variadic = std::integral_constant<bool,                          
                       !!BOOST_PP_TUPLE_SIZE(BOOST_PP_SEQ_ELEM(0, product))>; 
};