C++ 数组类型依赖于模板类型

Question

我有一个具有完整模板参数 N 的类。

template<unsigned int N>
class test {
}

现在我想要一个 std::vector，其整数类型尽可能小以容纳 N 位。

例如

class test<8> {
    std::vector<uint8_t> data;
}

class test<9> {
    std::vector<uint16_t> data;
}

class test<10> {
    std::vector<uint16_t> data;
}
...

N=1 到 N=64 是否有更好的方法？

Answer 1

使用条件怎么样？

#include <vector>
#include <cstdint>
#include <iostream>
#include <type_traits>

template <std::size_t N>
struct foo
 {
   static_assert( N < 65U, "foo 64 limit");

   using vType = typename std::conditional<
      (N < 9U), std::uint8_t,
        typename std::conditional< (N < 17U), std::uint16_t,
           typename std::conditional< (N < 33U), std::uint32_t, std::uint64_t
   >::type>::type>::type;

   std::vector<vType> data;
 };

int main()
 {
   static_assert( 1U == sizeof(foo<1>::vType), "!");
   static_assert( 1U == sizeof(foo<8>::vType), "!");
   static_assert( 2U == sizeof(foo<9>::vType), "!");
   static_assert( 2U == sizeof(foo<16>::vType), "!");
   static_assert( 4U == sizeof(foo<17>::vType), "!");
   static_assert( 4U == sizeof(foo<32>::vType), "!");
   static_assert( 8U == sizeof(foo<33>::vType), "!");
   static_assert( 8U == sizeof(foo<64>::vType), "!");

   // foo<65> f65; compilation error
 }

或者，以更优雅的方式(恕我直言)，您可以定义一个类型特征(selectTypeByDim，在下面的示例中)来选择列表中的第一个有用类型

#include <tuple>
#include <vector>
#include <cstdint>
#include <climits>
#include <type_traits>

template <std::size_t N, typename T,
   bool = (N <= sizeof(typename std::tuple_element<0U, T>::type)*CHAR_BIT)>
struct stbdH;

template <std::size_t N, typename T0, typename ... Ts>
struct stbdH<N, std::tuple<T0, Ts...>, true>
 { using type = T0; };

template <std::size_t N, typename T0, typename ... Ts>
struct stbdH<N, std::tuple<T0, Ts...>, false>
 { using type = typename stbdH<N, std::tuple<Ts...>>::type; };

template <std::size_t N, typename ... Ts>
struct selectTypeByDim : stbdH<N, std::tuple<Ts...>>
 { };

template <std::size_t N>
struct foo
 {
   static_assert( N < 65U, "foo 64 limit");

   using vType = typename selectTypeByDim<N,
         std::uint8_t, std::uint16_t, std::uint32_t, std::uint64_t>::type;

   std::vector<vType> data;
 };

int main()
 {
   static_assert( 1U == sizeof(foo<1U>::vType), "!");
   static_assert( 1U == sizeof(foo<CHAR_BIT>::vType), "!");
   static_assert( 2U == sizeof(foo<CHAR_BIT+1U>::vType), "!");
   static_assert( 2U == sizeof(foo<(CHAR_BIT<<1)>::vType), "!");
   static_assert( 4U == sizeof(foo<(CHAR_BIT<<1)+1U>::vType), "!");
   static_assert( 4U == sizeof(foo<(CHAR_BIT<<2)>::vType), "!");
   static_assert( 8U == sizeof(foo<(CHAR_BIT<<2)+1U>::vType), "!");
   static_assert( 8U == sizeof(foo<(CHAR_BIT<<3)>::vType), "!");

   //foo<(CHAR_BIT<<3)+1U> f65; compilation error
 }