is_incrementable.hpp代碼閱讀中的template實現細節推敲, 如代碼註釋:
namespace boost { namespace detail {
// is_incrementable<T> metafunction
//
// Requires: Given x of type T&, if the expression ++x is well-formed
// it must have complete type; otherwise, it must neither be ambiguous
// nor violate access.
// This namespace ensures that ADL doesn't mess things up.
namespace is_incrementable_
{
// a type returned from operator++ when no increment is found in the
// type's own namespace
struct tag {}; //測試函數的返回值用
// any soaks up implicit conversions and makes the following
// operator++ less-preferred than any other such operator that
// might be found via ADL.
struct any { template <class T> any(T const&); }; //測試函數的入參隱式類型轉換用
// 所有未定義++操作符的類型都會調用any的構造函數,進行隱式類型轉換,而後調用如下操作符函數
// 此時返回值爲操作符調用的返回值爲tag
tag operator++(any const&);
tag operator++(any const&,int);
// In case an operator++ is found that returns void, we'll use ++x,0
// tag類型的逗號表達式特殊處理,否則(tag, int)的返回值爲int類型,導致後續的測試函數匹配通過
tag operator,(tag,int);
// 定義逗號表達式,規避返回值爲void類型的情況
// 若某函數返回值爲void,如void ReturnVoid(),則下述測試代碼:
// std::cout << (ReturnVoid(), 10) << std::endl;的輸出爲:10
// 其中表達式前後的括號是必須的
# define BOOST_comma(a,b) (a,b)
// two check overloads help us identify which operator++ was picked
// check_測試函數,使用編譯器的SFINAE技巧,篩選不支持操作符的類型
char (& check_(tag) )[2]; //check_函數返回值爲char[2]-char類型數組引用,注意函數返回值不能爲數組類型
template <class T>
char check_(T const&);
template <class T>
struct impl
{
static typename boost::remove_cv<T>::type& x;
BOOST_STATIC_CONSTANT(
bool
//1. 需要逗號表達時包裹一下,否則會在++x返回值爲void類型時,
//check函數的入參爲void const&類型,不合法參數導致編譯失敗
//2. check_返回char爲測試通過,返回爲char&[2]測試失敗
, value = sizeof(is_incrementable_::check_(BOOST_comma(++x,0))) == 1
);
};
template <class T>
struct postfix_impl
{
static typename boost::remove_cv<T>::type& x;
BOOST_STATIC_CONSTANT(
bool
, value = sizeof(is_incrementable_::check_(BOOST_comma(x++,0))) == 1
);
};
# if defined(BOOST_MSVC)
# pragma warning(pop)
# endif
}
//去掉宏定義,避免頭文件包含中同名宏定義的覆蓋
# undef BOOST_comma
template<typename T>
struct is_incrementable :
public boost::integral_constant<bool, boost::detail::is_incrementable_::impl<T>::value>
{
//看了下枚舉展開,定義了rebind函數,具體什麼用途不是很清楚
BOOST_MPL_AUX_LAMBDA_SUPPORT(1,is_incrementable,(T))
};
template<typename T>
struct is_postfix_incrementable :
public boost::integral_constant<bool, boost::detail::is_incrementable_::postfix_impl<T>::value>
{
BOOST_MPL_AUX_LAMBDA_SUPPORT(1,is_postfix_incrementable,(T))
};
} // namespace detail
} // namespace boost
# include <boost/type_traits/detail/bool_trait_undef.hpp>
所以整體推導推導策略爲,若測試類型有操作符++的實現:
- 無論operator++返回值是何種類型,包括void,均認爲是支持;
- 隱式轉換後的類型支持operator++(在ADL的類型匹配中優先級高於any的模板構造函數即可),均認爲是支持。