C++ C+中的编译时数组选择+;
我有一个类,它保存一个基于编译时参数的数组(用于过滤器)。例如:C++ C+中的编译时数组选择+;,c++,templates,c++17,constexpr,template-specialization,C++,Templates,C++17,Constexpr,Template Specialization,我有一个类,它保存一个基于编译时参数的数组(用于过滤器)。例如: template<class Real, unsigned N> class foo { public: // stuff ... private: static const constexpr std::array<Real, unsigned N> m_h; }; 如果N=4和Real=float,我想 m_h = {0.4829629f, 0.8365163f, 0.2241438f, -
template<class Real, unsigned N>
class foo {
public:
// stuff ...
private:
static const constexpr std::array<Real, unsigned N> m_h;
};
如果N=4
和Real
=float
,我想
m_h = {0.4829629f, 0.8365163f, 0.2241438f, -0.1294095f};
如果N=6
,并且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};
实现这一目标最优雅的语法是什么?我所发现的最接近的是Boost,它通过首先按位数对
Real
类型进行分类,并可转换为float、double和long double来实现这一目标。然后介绍一个foo\u detail
类,该类提供函数get\u constants()
,并选择运行时所需的常量。Boost当然支持许多编译器及其不同的C++11功能,因此我觉得使用(比如)C++17可能会有一个更具表现力的解决方案。我不知道您到底想要什么,但。。。我想您可以初始化m_h
调用constexpr
模板函数,并对其进行完全专门化
我是说。。。您可以编写foo()
,如下所示
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;
以下是完整的编译示例(使用简化的foo
)
“我不知道你到底想要什么,但是……”这正是我想要的。谢谢
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;
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
#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;
}
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 ) ...
}
}