internal/meta.h: add some template meta functions

 * Add/RemoveConst
 * IsSame, IsConst, IsMoreConst
 * Enable/DisableIf (including helper macro)

include/rapidjson/internal/meta.h

+#ifndef RAPIDJSON_INTERNAL_META_H_
+#define RAPIDJSON_INTERNAL_META_H_
+
+//@cond RAPIDJSON_INTERNAL
+namespace rapidjson {
+namespace internal {
+
+template <int N> struct IntegralC { enum { Value = N }; };
+template <bool Cond> struct BoolType : IntegralC<Cond> {};
+struct TrueType : BoolType<true> {};
+struct FalseType : BoolType<false> {};
+
+template <typename T> struct AddConst { typedef const T Type; };
+template <typename T> struct RemoveConst { typedef T Type; };
+template <typename T> struct RemoveConst<const T> { typedef T Type; };
+
+template <bool Condition, typename T1, typename T2> struct SelectIfCond;
+template <typename T1, typename T2> struct SelectIfCond<true,T1,T2> { typedef T1 Type; };
+template <typename T1, typename T2> struct SelectIfCond<false,T1,T2> { typedef T2 Type; };
+
+template <typename Condition, typename T1, typename T2>
+struct SelectIf : SelectIfCond<Condition::Value,T1,T2> {};
+
+template <bool Constify, typename T>
+struct MaybeAddConst : SelectIfCond<Constify, const T, T> {};
+
+template <typename T, typename U> struct IsSame { enum { Value = false }; };
+template <typename T> struct IsSame<T,T> { enum { Value = true }; };
+
+template <typename T> struct IsConst { enum { Value = false }; };
+template <typename T> struct IsConst<const T> { enum { Value = true }; };
+
+template <typename CT, typename T>
+struct IsMoreConst {
+	enum { Value =
+		( IsSame< typename RemoveConst<CT>::Type, typename RemoveConst<T>::Type>::Value
+		&& ( IsConst<CT>::Value >= IsConst<T>::Value ) )
+	};
+};
+
+template <bool Condition, typename T = void> struct EnableIfCond;
+template <typename T> struct EnableIfCond<true, T> { typedef T Type; };
+template <typename T> struct EnableIfCond<false, T> { /* empty */ };
+
+template <bool Condition, typename T = void>
+struct DisableIfCond : EnableIfCond<not Condition, T> {};
+
+template <typename Condition, typename T = void>
+struct EnableIf : EnableIfCond<Condition::Value, T> {};
+
+template <typename Condition, typename T = void>
+struct DisableIf : DisableIfCond<Condition::Value, T> {};
+
+// SFINAE helpers
+struct SfinaeResultTag {};
+template <typename T> struct RemoveSfinaeFptr {};
+template <typename T> struct RemoveSfinaeFptr<SfinaeResultTag&(*)(T)> { typedef T Type; };
+
+#define RAPIDJSON_REMOVEFPTR_(type) \
+	typename ::rapidjson::internal::RemoveSfinaeFptr \
+		< ::rapidjson::internal::SfinaeResultTag&(*) type>::Type
+
+#define RAPIDJSON_ENABLEIF(cond) \
+	typename ::rapidjson::internal::EnableIf \
+		<RAPIDJSON_REMOVEFPTR_(cond)>::Type * = NULL
+
+} // namespace internal
+} // namespace rapidjson
+//@endcond
+
+#endif // RAPIDJSON_INTERNAL_META_H_