C ++奇怪的std :: bad_alloc异常

Question

So I have the following code:

#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
const int MAXN = 1000000;
int isNotPrime[MAXN];
vector<int> primes;

void sieve()
{
    for(int i = 2; i <= sqrt(MAXN); ++i)
    {
        if(isNotPrime[i]) continue;
        for(int j = i*i; j <= MAXN; j += i)
        {
            isNotPrime[j] = true;
        }
    }
    for(int i = 2; i <= MAXN; ++i)
    {
        if(!isNotPrime[i])
        {
            primes.push_back(i);
        }
    }
}

int main()
{
    ios::sync_with_stdio(false);
    sieve();
    return 0;
}

What I cannot understand is why my program throws a std::bad_alloc exception when it executes. Even more mind-boggling is that when I swap the lines int isNotPrime[MAXN]; and vector<int> primes; the programs executes as intended. Swapped like this:

vector<int> primes;
int isNotPrime[MAXN];

Answer 1

The problem is here:

for(int i = 2; i <= MAXN; ++i)

The check should be i < MAXN instead. (Or, make the array have size MAXN + 1 .)

At some point, the isNotPrime[MAXN] = true; executes, which overflows the bounds of the array, causing undefined behaviour . In practice, this overwrites some internal field of the next variable ( primes ), which confuses the std::vector implementation, probably causing it to request a lot of memory.

This also explains why switching the variable order "fixes" it, because now you're scribbling over something else instead of primes .