我正在尝试实现高斯-勒让德求积,并且我想要一个模板化函数 将点数作为模板参数。 现在我有这个:
template<int number_of_quadrature_points>
double gaussian_quadrature_integral_core(double (*f)(double), double from, double to){
double scaling_factor = (to-from)/2;
double average_factor = (from+to)/2;
std::array<double, number_of_quadrature_points> factors;
std::array<double, number_of_quadrature_points> points;
if constexpr(number_of_quadrature_points == 2){
factors = {1, 1};
points = {-1.0/sqrt(3), 1.0/sqrt(3)};
}
if constexpr(number_of_quadrature_points == 3){
factors = {5.0/9.0, 8.0/9.0, 5.0/9.0};
points = {-sqrt(3.0/5.0), 0, sqrt(3.0/5.0)};
}
double sum = 0;
for(int i = 0; i < number_of_quadrature_points; i++){
sum += factors.at(i)*((*f)(scaling_factor*points.at(i)+average_factor));
}
sum *= scaling_factor;
return sum;
}
如您所见,当模板参数发生变化时,不仅数组大小发生变化,而且内容也发生变化,但对于给定的大小,内容是众所周知的。出于这个原因,我认为如果 std::arrays 是 const static 会更好,因为该函数被调用很多次。
现在我只能使用 if constexpr 来声明数组,但是如何使用它来定义和声明数组,以便它在 if constexpr 范围之外可见,并且数组只定义一次?
最佳答案
添加两个辅助函数就足够了(如果您使用的是 C++20):
template<unsigned N>
constexpr auto init_factors() {
std::array<double, N> rv;
if constexpr(N == 2){
rv = {1., 1.};
} else {
rv = {5.0/9.0, 8.0/9.0, 5.0/9.0};
}
return rv;
}
template<unsigned N>
constexpr auto init_points() {
std::array<double, N> rv;
if constexpr(N == 2){
rv = {-1.0/std::sqrt(3.), 1.0/std::sqrt(3.)};
} else {
rv = {-std::sqrt(3.0/5.0), 0, std::sqrt(3.0/5.0)};
}
return rv;
}
template<unsigned number_of_quadrature_points>
double gaussian_quadrature_integral_core(double (*f)(double), double from,
double to)
{
static constexpr auto factors = init_factors<number_of_quadrature_points>();
static constexpr auto points = init_points<number_of_quadrature_points>();
[...]
为了防止使用错误的点数,您可以添加 static_assert
template<unsigned number_of_quadrature_points>
double
gaussian_quadrature_integral_core(double (*f)(double), double from,
double to)
{
static_assert(number_of_quadrature_points==2||number_of_quadrature_points==3);
...或者如果您想稍后进行特化,则可以阻止使用 SFINAE 进行匹配:
#include <type_traits>
template<unsigned number_of_quadrature_points>
std::enable_if_t<number_of_quadrature_points==2||number_of_quadrature_points==3,
double>
gaussian_quadrature_integral_core(double (*f)(double), double from,
double to)
{
关于c++ - 使用 constexpr 定义并声明 const std::array if,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/65341949/