C++ 类 : calling private function in for-loop from public function

Question

给定一系列大数

candidates=[6541367000, 6541368000)

找出那些只能被 2 个质数分解的。

我写了一个 C++ 类:

vector<long long>& prime_fac(N)

返回 N 个素数的 vector ，并且

void print2PrimeFactoriation(vector<long long>& candidates)

仅返回具有 2 个主要因素的候选人。

#include <cmath>
#include <iostream>
#include <vector>
using namespace std;

class primeFactorization {
private:
    //vector<long long> prime_vec;
    long long upper_limit;
    vector<long long> res;

    vector<long long>& prime_fac(long long N) {
        vector<long long> prime_vec;
        if (N < 2) {
            cout << "N=" << N << " cannot be factorized by prime numbers" << endl;
        }
        else {
            upper_limit = (long long)sqrt(N);
            while (N%2 == 0) {
                prime_vec.emplace_back(2);
                N /= 2;
            }
            for (long long prime=3; prime<=upper_limit; prime+=2) {
                while (N%prime == 0) {
                    prime_vec.emplace_back(prime);
                }
            }
            prime_vec.emplace_back(N);
        }
        return prime_vec;
    }

public:
    void print2PrimeFactoriation(vector<long long>& candidates) {
        for (long long candidate : candidates) {
            res = prime_fac(candidate);
            cout << candidate << endl;
            if (res.size() == 2) {
                cout << candidate << " can be factorized by 2 prime numbers (";
                cout << res[0] << ", " << res[1] << ")\n";
            }
        }
    }
};


int main() {
    primeFactorization sol;

    vector<long long> candidates;
    for (long long candidate=6541367001; candidate<6541368000; candidate+=2) {
        candidates.emplace_back(candidate);
    }

    sol.print2PrimeFactoriation(candidates);

    return 0;
}

奇怪的是，我收到了错误信息:

terminate called after throwing an instance of 'std::bad_alloc'                                                                                                                      
  what():  std::bad_alloc                                                                                                                                                            
Aborted 

我什至尝试将输入 vector 候选减少为仅包含 1 个元素，但同样的错误仍然存​​在。

错误是什么？我该怎么做才能解决？另外，它是进行质因数分解的最有效方法吗？

Answer 1

您正在返回对局部变量的引用 prime_vec在函数中 vector<long long>& prime_fac(long long N) .

vector<long long>& prime_fac(long long N) {
    vector<long long> prime_vec;
    ...
    ... 
    return prime_vec;
}

局部变量的生命周期prime_vec这里仅限于函数的范围。在范围退出后尝试使用它会导致未定义的行为。

像 GCC and Clang issue a warning 这样的编译器当你这样做时:

warning: reference to stack memory associated with local variable 'prime_vec' returned [-Wreturn-stack-address]
        return prime_vec;
               ^~~~~~~~~