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C++中编译时数组的选择

template-specialization constexpr c++17 templates c++

user14717 · 技术社区 · 6 年前

template<class Real, unsigned N>
class foo {
public:
  // stuff ...
private:
   static const constexpr std::array<Real, unsigned N> m_h;
};

例如,如果 N=4 和 Real = double ,我想 m_h

m_h = {0.4829629131445341,  0.8365163037378079,  0.2241438680420133, -0.129409522551260};

如果 N=4个 真实的 = float ,我想

m_h = {0.4829629f, 0.8365163f, 0.2241438f, -0.1294095f};

如果 N=6 Real=double ,我希望数字完全不同:

m_h = {0.332670552950082615998, 0.806891509311092576494, 0.45987750211849157009, -0.1350110200102545886963899, -0.0854412738820266616928191, 0.0352262918857095366027};

Gaussian quadrature 真实的 按位数键入,可转换为浮点、双精度和长双精度。然后介绍了一个 foo_detail 提供函数的类 get_constants() 并选择运行时所需的常量。当然,Boost支持许多编译器及其不同的C++11功能,所以我觉得可能会有一个使用(比如)C++17的更具表现力的解决方案。

1 回复 | 直到 6 年前

StephenA shashi009 6 年前

我不知道你到底想要什么但是。。。我想你可以初始化 m_h 呼叫 constexpr 模板函数并对其进行完全专门化。

foo() 如下

template <typename Real, std::size_t N>
class foo
 {
   private:
      static constexpr std::array<Real, N> m_h { bar<Real, N>() };
 };

template <typename Real, std::size_t N>
constexpr std::array<Real, N> foo<Real, N>::m_h;

写一套 bar()

template <typename Real, std::size_t N>
constexpr std::array<Real, N> bar ();

template <>
constexpr std::array<double, 4u> bar<double, 4u> ()
 { return { {0.4829629131445341,  0.8365163037378079,
             0.2241438680420133, -0.129409522551260} }; }

template <>
constexpr std::array<float, 4u> bar<float, 4u> ()
 { return { {0.4829629f, 0.8365163f, 0.2241438f, -0.1294095f} }; }

template <>
constexpr std::array<double, 6u> bar<double, 6u> ()
 { return { { 0.332670552950082615998,      0.806891509311092576494,
              0.45987750211849157009,      -0.1350110200102545886963899,
             -0.0854412738820266616928191,  0.0352262918857095366027} }; }

// as many `bar()` specializations as you want

下面是一个完整的编译示例(带有一个简化的 foo

#include <array>
#include <iostream>

template <typename Real, std::size_t N>
constexpr std::array<Real, N> bar ();

template <>
constexpr std::array<double, 4u> bar<double, 4u> ()
 { return { {0.4829629131445341,  0.8365163037378079,
             0.2241438680420133, -0.129409522551260} }; }

template <>
constexpr std::array<float, 4u> bar<float, 4u> ()
 { return { {0.4829629f, 0.8365163f, 0.2241438f, -0.1294095f} }; }

template <>
constexpr std::array<double, 6u> bar<double, 6u> ()
 { return { { 0.332670552950082615998,      0.806891509311092576494,
              0.45987750211849157009,      -0.1350110200102545886963899,
             -0.0854412738820266616928191,  0.0352262918857095366027} }; }

template <typename Real, std::size_t N>
struct foo
 {
   static constexpr std::array<Real, N> m_h { bar<Real, N>() };
 };

template <typename Real, std::size_t N>
constexpr std::array<Real, N> foo<Real, N>::m_h;

int main ()
 {
   for ( auto f : foo<double, 4u>::m_h )
      std::cout << f << ' ';

   std::cout << std::endl;

   for ( auto f : foo<float, 4u>::m_h )
      std::cout << f << ' ';

   std::cout << std::endl;

   for ( auto f : foo<double, 6u>::m_h )
      std::cout << f << ' ';

   std::cout << std::endl;
 }

或者,如果你不想发展 条形图() 条形图() if constexpr 如下

template <typename Real, std::size_t N>
constexpr std::array<Real, N> bar ()
 {
   if constexpr ( std::is_same<long double, Real>::value )
    {
      if constexpr ( 4u == N )
         return { /* something */ };
      else if constexpr ( 6u == N )
         return { /* something */ };
      // else if constexpr ( ?? == N ) ...
    }
   else if constexpr ( std::is_same<double, Real>::value )
    { 
      if constexpr ( 4u == N )
         return { {0.4829629131445341,  0.8365163037378079,
                   0.2241438680420133, -0.129409522551260} }; 
      else if constexpr ( 6u == N )
         return { { 0.332670552950082615998,      0.806891509311092576494,
                    0.45987750211849157009,      -0.1350110200102545886963899,
                   -0.0854412738820266616928191,  0.0352262918857095366027} }; 
      // else if constexpr ( ?? == N ) ...
    }
   else if constexpr ( std::is_same<float, Real>::value )
    { 
      if constexpr ( 4u == N )
         return { {0.4829629f, 0.8365163f, 0.2241438f, -0.1294095f} };
      else if constexpr ( 6u == N )
         return { /* something */ };
      // else if constexpr ( ?? == N ) ...
    }
 }