在模板参数包中使用以下方法确定常用数字类型的最佳方法是：
可变参数模板（best_common_numeric_type）可以这样使用：
template<typename... NumericTypes>
auto some_numeric_func(const NumericTypes&...)
-> typename best_common_numeric_type<NumericTypes...>::type;
并拥有如下实例：
[1] best_common_numeric_type<long, unsigned long, float, double, int>::type = double
[2] best_common_numeric_type<unsigned int, unsigned long>::type = unsigned long
[3] best_common_numeric_type<signed int, signed long>::type = signed long
[4] best_common_numeric_type<signed int, unsigned int>::type = signed long
[5] best_common_numeric_type<signed int, unsigned long>::type = int128_t (maybe)
例如，在[4]例中，::type必须是signed long，因为signed int无法保留unsigned int而没有溢出的风险，反之亦然unsigned int {1}}无法保留signed int而没有下溢风险。
同样适用于[5]，但现在signed long不再足够，因为它无法保持unsigned long而没有溢出的风险。
（实施可能是data model具体的，但你明白了。）
那么在C ++ 11中实现这一目标的最佳方式是什么呢？
答案 0 :(得分：6)
您可以使用Boost Integer选择合适的案例。
暂时忽略非整数元素类型的情况，这里是对建议案例的快速测试（GCC没有出现int128_t）：
<强> Live on Coliru
#include <boost/mpl/vector.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/max_element.hpp>
#include <boost/integer.hpp>
#include <limits>
using namespace boost;
namespace best_fit_
{
// wrappers around Boost Integer http://www.boost.org/doc/libs/1_54_0/libs/integer/doc/html/boost_integer/integer.html#boost_integer.integer.sized
template <bool is_signed, int bin_digits> struct select_int;
template <int bin_digits> struct select_int<true, bin_digits> {
using type = typename boost::int_t<bin_digits + 1>::least;
};
template <int bin_digits> struct select_int<false, bin_digits> {
using type = typename boost::uint_t<bin_digits>::least;
};
// query helper
struct digits {
template <typename I> using apply = mpl::int_<std::numeric_limits<I>::digits>;
};
}
template <typename... I>
struct best_common_integral
{
private:
using Ints = mpl::vector<I...>;
using Bits = typename mpl::transform<Ints, best_fit_::digits>::type;
template <typename J>
struct is_signed { static constexpr bool value = std::numeric_limits<J>::is_signed; };
using max = typename mpl::deref<typename mpl::max_element<Bits>::type>::type;
// sigh, there is no `mpl::any`, AFAICT
using sign = typename mpl::fold<
Ints,
mpl::bool_<false>,
mpl::if_<is_signed<mpl::_2>, mpl::bool_<true>, mpl::_1>
>::type;
public:
using type = typename best_fit_::select_int<sign::value, max::value>::type;
};
#include <typeinfo>
#include <iostream>
#include <cassert>
int main()
{
using case1 = best_common_integral<long, unsigned long, float, double, int>;
using case2 = best_common_integral<unsigned int, unsigned long>;
using case3 = best_common_integral<signed int, signed long>;
using case4 = best_common_integral<signed int, unsigned int>;
using case5 = best_common_integral<signed int, unsigned long>;
//assert(typeid(case1::type) == typeid(double));
assert(typeid(case2::type) == typeid(unsigned long));
assert(typeid(case3::type) == typeid(signed long));
assert(typeid(case4::type) == typeid(signed long));
//assert(typeid(case5::type) == typeid(int128_t (maybe)));
}
答案 1 :(得分：6)
我参加派对有点晚了， 这是我没有Boost的解决方案：
#include <type_traits>
#include <cstdint>
template<class I, bool Signed> struct mk_signed { typedef I type; };
template<> struct mk_signed<uint8_t , true> { typedef int16_t type; };
template<> struct mk_signed<uint16_t, true> { typedef int32_t type; };
template<> struct mk_signed<uint32_t, true> { typedef int64_t type; };
template<> struct mk_signed<uint64_t, true> { typedef int64_t type; };
template <typename... Ts> struct best_common_numeric_type;
template <typename T> struct best_common_numeric_type<T> { typedef T type; };
template <typename T, typename... Ts>
struct best_common_numeric_type<T, Ts...> {
typedef typename best_common_numeric_type<Ts...>::type TS;
typedef typename std::conditional < (sizeof(T) > sizeof(TS)), T, TS>::type bigger_integral;
constexpr static bool fp = std::is_floating_point<T>::value || std::is_floating_point<TS>::value;
constexpr static bool have_signed = !fp && (std::is_signed<T>::value || std::is_signed<TS>::value);
typedef typename std::conditional <
fp,
typename std::common_type<T,TS>::type,
typename mk_signed<bigger_integral,have_signed>::type
>::type type;
};
答案 2 :(得分：4)
据我所知，这没有什么标准，但可以做到： http://coliru.stacked-crooked.com/view?id=c6aa42345f91ab51d745d56573b15a04-4f34a5fd633ef9f45cb08f8e23efae0a
首先是“思想家”结构。
template<bool isfloat, bool negative> struct best_numeric_type
{typedef long double type;};
template<> struct best_numeric_type<false, true>
{typedef long long type;};
template<> struct best_numeric_type<false, false>
{typedef unsigned long long type;};
然后是基本情况：
template<class T> struct best_common_numeric_type1 {
static const bool isfloat=false;
static const bool negative=false;
typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<> struct best_common_numeric_type1<char> {
static const bool isfloat=false;
static const bool negative=true;
typedef typename best_numeric_type<isfloat, negative>::type type;
};//copy-paste for signed char, short, int, long, and long long.
template<> struct best_common_numeric_type1<float> {
static const bool isfloat=true;
static const bool negative=false;
typedef typename best_numeric_type<isfloat, negative>::type type;
};//copy-paste for double and long double.
然后是加入者：
template<class First, class Second>
struct best_common_numeric_type2 {
static const bool isfloat = best_common_numeric_type1<First>::isfloat | best_common_numeric_type1<Second>::isfloat;
static const bool negative = best_common_numeric_type1<First>::negative | best_common_numeric_type1<Second>::negative;
typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third>
struct best_common_numeric_type3 {
static const bool isfloat = best_common_numeric_type2<First, Second>::isfloat | best_common_numeric_type1<Third>::isfloat;
static const bool negative = best_common_numeric_type2<First, Second>::negative | best_common_numeric_type1<Third>::negative;
typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third, class Fourth>
struct best_common_numeric_type4 {
static const bool isfloat = best_common_numeric_type3<First, Second, Third>::isfloat | best_common_numeric_type1<Fourth>::isfloat;
static const bool negative = best_common_numeric_type3<First, Second, Third>::negative | best_common_numeric_type1<Fourth>::negative;
typedef typename best_numeric_type<isfloat, negative>::type type;
};
template<class First, class Second, class Third, class Fourth, class Fifth>
struct best_common_numeric_type5 {
static const bool isfloat = best_common_numeric_type4<First, Second, Third, Fourth>::isfloat | best_common_numeric_type1<Fifth>::isfloat;
static const bool negative = best_common_numeric_type4<First, Second, Third, Fourth>::negative | best_common_numeric_type1<Fifth>::negative;
typedef typename best_numeric_type<isfloat, negative>::type type;
};
最后一个测试：
#include <typeinfo>
#include <iostream>
void printer(long double) {std::cout << "long double\n";}
void printer(unsigned long long) {std::cout << "ull\n";}
void printer(long long) {std::cout << "ll\n";}
void printer(...) {std::cout << "else\n";}
int main() {
printer(best_common_numeric_type5<long, unsigned long, float, double, int>::type());
printer(best_common_numeric_type2<unsigned int, unsigned long>::type());
printer(best_common_numeric_type2<signed int, signed long>::type());
printer(best_common_numeric_type2<signed int, unsigned int>::type());
printer(best_common_numeric_type2<signed int, unsigned long>::type());
printer(best_common_numeric_type2<float, char>::type());
}
结果：
long double
ull
ll
ll
ll
long double