问题
C++11 provides two type trait template classes: std::is_integer and std::is_integral. However, I cannot tell the differences between them.
What type, say T, can make std::is_integer<T>::value true and make std::is_integral<T>::value false?
回答1:
std::is_integer<T> does not exist.
That being said, std::numeric_limits<T>::is_integer does exist.
I'm not aware of any significant difference between std::numeric_limits<T>::is_integer and std::is_integral<T>. The latter was designed much later and became standard in C++11, whereas the former was introduced in C++98.
回答2:
There is no type T that has different results for std::is_integral<T>::value and std::numeric_limits<T>::is_integer. To quote the draft Standard:
3.9.1 Fundamental types [basic.fundamental]
7 Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types. A synonym for integral type is integer type.[...]
18.3.2.4 numeric_limits members [numeric.limits.members]
static constexpr bool is_integer;
17 True if the type is integer.
20.9.4.1 Primary type categories [meta.unary.cat] (table 47)
template <class T> struct is_integral;
T is an integral type (3.9.1)
回答3:
std::is_integral_v<T> will only return true for built-in integers.
The standard allows std::numeric_limits<T>::is_integer to be specialized and return true for custom integral types like boost::multiprecion::cpp_int.
回答4:
The difference is that std::is_integral<T> will only recognize decimal integers including bool char char16_t char32_t wchar_t short int long long long. While std::numeric_limits<T>::is_integer will recognize all of those aswell as float double. Look at these two pages for more information: is_integer, is_integral
来源：https://stackoverflow.com/questions/14676574/differences-between-stdis-integer-and-stdis-integral