boost之 remove_cv

从remove_cv名字中，可以看出这个traits与const, volatile有关，具体什么作用，还是看看boost的代码吧
注意boost\type_traits中的定义
BOOST_TT_AUX_TYPE_TRAIT_DEF1(remove_cv,T,typename boost::detail::rvalue_ref_filter_rem_cv<T>::type)
BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_1(typename T,remove_cv,T&,T&)
#if !defined(BOOST_NO_ARRAY_TYPE_SPECIALIZATIONS)
BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,remove_cv,T const[N],T type[N])
BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,remove_cv,T volatile[N],T type[N])
BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_2(typename T,std::size_t N,remove_cv,T const volatile[N],T type[N])
#endif
把宏展开，得到以下结果
//BOOST_TT_AUX_TYPE_TRAIT_DEF1(remove_cv,T,typename boost::detail::rvalue_ref_filter_rem_cv<T>::type)
//1
template< typename T > struct remove_cv
{
public:
typedef typename boost::detail::rvalue_ref_filter_rem_cv<T>::type type;
BOOST_MPL_AUX_LAMBDA_SUPPORT(1,remove_cv,(T))
};

BOOST_TT_AUX_TEMPLATE_ARITY_SPEC(1,remove_cv)

//#define BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_1(typename T, remove_cv,T&,T&)
//2
template< typename T> struct remove_cv<T&>
{
public:
typedef T&type;
typedef int& TAG;
};
/**/

//#define BOOST_TT_AUX_TYPE_TRAIT_PARTIAL_SPEC1_2(param1,param2,trait, T const[N], result)
//3
template< typename T, std::size_t N> struct remove_cv<T const[N]>
{
public:
typedef T type[N];
};
//4
template< typename T, std::size_t N> struct remove_cv<T volatile[N]>
{
public:
typedef T type[N];
};
//5
template< typename T, std::size_t N> struct remove_cv<const volatile[N]>
{
public:
typedef T type[N];
};
可知，1是remove_cv的范化版本，2-5分别是对remove_cv的当T为T&, T const[N], T volatile[N], const Volatile[N]的特化版本，实际上，从上面可以看出,如果T是一个引用，则remove_cv<T>::type 还是一个引用，没有变，至于const 数组,volatile数组，则remove_cv<T>::type是T[N]
最后，来看看remove_cv范型版本,BOOST_MPL_AUX_LAMBDA_SUPPORT(1,remove_cv,(T)) 和BOOST_TT_AUX_TEMPLATE_ARITY_SPEC(1,remove_cv) 忽略，rvalue_ref_filter_rem_cv代码如下
template <class T>
struct rvalue_ref_filter_rem_cv
{
typedef typename boost::detail::cv_traits_imp<T*>::unqualified_type type;
};
template <typename T> struct cv_traits_imp {};

template <typename T>
struct cv_traits_imp<T*>
{
BOOST_STATIC_CONSTANT(bool, is_const = false);
BOOST_STATIC_CONSTANT(bool, is_volatile = false);
typedef T unqualified_type;
};

template <typename T>
struct cv_traits_imp<const T*>
{
BOOST_STATIC_CONSTANT(bool, is_const = true);
BOOST_STATIC_CONSTANT(bool, is_volatile = false);
typedef T unqualified_type;
};

template <typename T>
struct cv_traits_imp<volatile T*>
{
BOOST_STATIC_CONSTANT(bool, is_const = false);
BOOST_STATIC_CONSTANT(bool, is_volatile = true);
typedef T unqualified_type;
};

template <typename T>
struct cv_traits_imp<const volatile T*>
{
BOOST_STATIC_CONSTANT(bool, is_const = true);
BOOST_STATIC_CONSTANT(bool, is_volatile = true);
typedef T unqualified_type;
};
所以remove_cv<T>::type 就是去掉const, volatile修饰符后的类型