[英]Filling an std::array using math formula in compile time
I want to fill a constexpr std::array in compile time using a math function. Is it possible in an easy way?我想在编译时使用数学 function 填充一个 constexpr std::array。是否有可能以简单的方式进行?
I found this solution: C++11: Compile Time Calculation of Array .我找到了这个解决方案: C++11: Compile Time Calculation of Array 。 However, is there any other modern solution using only std?但是,还有其他仅使用 std 的现代解决方案吗? This one seems too confusing for me.这对我来说似乎太混乱了。
int main()
{
// This is the array I need to fill
constexpr std::array<double, 100000> elements;
for (int i=0; i!=100000; ++i)
{
// Each element is calculated using its position with long exponential maths.
elements[i] = complexFormula(i); // complexFormula is constexpr
}
double anyVal = elements[43621];
// ...
}
Here's a non-confusing approach: wrap the calculation in a function: 这是一个非混淆的方法:将计算包装在一个函数中:
template <int N>
constexpr std::array<double, N> generate()
{
std::array<double, N> arr{};
for (int i = 0; i < N; ++i)
arr[i] = complexFormula(i);
return arr;
}
Usage example: 用法示例:
constexpr std::array<double, 10000> arr = generate<10000>();
This works because, since C++14, loops are allowed in a constexpr function, and variables can be modified as long as their lifetime starts within the evaluation of the constant expression. 这是有效的,因为从C ++ 14开始,constexpr函数中允许循环,并且只要它们的生命周期在常量表达式的计算中开始,就可以修改变量。
In c++14 you also have the possibility of using std::index_sequence
for your purpose:在 c++14 中,您还可以使用std::index_sequence
来达到您的目的:
template <std::size_t... I>
constexpr std::array<double, sizeof...(I)> generate_impl(std::index_sequence<I..>) noexcept
{
return { complexFormula(I)... };
}
template <std::size_t N>
constexpr std::array<double, N> generate() noexcept
{
return generate_impl(std::make_index_sequence<N>{});
}
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