多成员Construpr结构初始化 [英] Multiple Member Constexpr Struc Initialization
问题描述
我是constexpr的新手,但是,我认为我正在练习的问题很适合编译时计算.这不是为了做家庭作业或任何事情,而只是练习现代"练习.C ++.
I am new to constexpr, however, I believe that problem I am practicing is a good fit compile-time calculations. This is not for homework or anything, just practicing "Modern" C++.
这是我到目前为止所拥有的:标题(prime_app_lib.hpp):
Here is what I have so far: Header (prime_app_lib.hpp):
// #pragma once for modern header files (.hpp)
#pragma once
#include <math.h>
static constexpr unsigned max_prime = 10000U;
// check out this link for the number of primes: https://primes.utm.edu/howmany.html
static constexpr unsigned prime_array_size = 1230U;
constexpr unsigned get_nth_prime(unsigned n);
我正在尝试创建一个const数组(在编译时构造),我可以使用get_nth_prime(n)访问它.您可以看到我尝试使用所选结构实现的算法.无论我尝试了什么,都无法同时初始化isPrime和primeValue.我愿意接受任何建议,尤其是全面重写.我只希望能遵循良好和现代的C ++进行工作.
I am trying to create a const array (constructed at compile time) that I can access with get_nth_prime(n). You can see the algorithm I am trying to implement with the structure that I chose. No matter what I tried, I can't initialize both isPrime and primeValue. I am open to any suggestions, especially a full rewrite. I would just like something that works following good and modern C++.
#include "prime_app_lib.hpp"
#include <iostream>
struct PrimeData {
bool isPrime[max_prime];
unsigned primeValue[prime_array_size];
// Constructor for the data using SieveOfEratosthenes
// https://www.geeksforgeeks.org/program-to-find-the-nth-prime-number/
constexpr PrimeData() {
for (auto b : PrimeData::isPrime) {
b = true;
}
for (auto p : PrimeData::primeValue) {
p = 0;
}
for (int p = 2; p * p < max_prime; p++)
{
// If IsPrime[p] is not changed, then it is a prime
if (PrimeData::isPrime[p] == true)
{
// Update all multiples of p greater than or
// equal to the square of it
// numbers which are multiple of p and are
// less than p^2 are already been marked.
for (int i = p * p; i < max_prime; i += p)
PrimeData::isPrime[i] = false;
}
}
// Store all prime numbers
int n = 0;
for (int p = 2; p < max_prime; p++) {
if (PrimeData::isPrime[p] && n < prime_array_size)
PrimeData::primeValue[n] = static_cast<unsigned>(p);
}
}
};
// this was just to make sure I can get some constexpr compiling
constexpr unsigned get_nth_prime(unsigned n) {
return unsigned(prime_array_size);
}
int main() {
std::cout << prime_array_size << "\n";
constexpr unsigned test1 = get_nth_prime(2);
static_assert(test1 == prime_array_size, "test1");
}
推荐答案
问题是进入构造函数主体之前,需要初始化所有成员.因此,您可以这样做:
The issue is all your members need to be initialized before entering the constructor body. So you could do:
constexpr PrimeData() : isPrime{} , primeValue{} {
// ...
}
或简单地:
struct PrimeData {
bool isPrime[max_prime]{};
unsigned primeValue[prime_array_size]{};
// ...
};
这是一个演示.
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