std::bad_alloc thrown in short primes program

Question

I quickly wrote a primes program that uses the seive of eratosthenes. I get an appcrash when I run the program. I debugged it and it told me std::bad_alloc. The debugger will not tell me which line of my code the exception occured on but tells me which line of system code. My source is as follows. I am somewhat new to c++.

#include <iostream>
#include <vector>
#include <string>
#include <math.h>

using namespace std;

int main(int argc, char* argv[])
{
    unsigned long long numt;
    if(argc < 2) {
        cout << "Usage: "<<argv[0]<<" <primes until...>" << endl;
        return 1;
    }

    else if(atol(argv[1])) {
        cout << "Usage: "<<argv[0]<<" <primes until...>" << endl;
        return 1;
    }
    numt = atol(argv[1]);
    vector<bool> primes(numt);

    for each(bool b in primes) b = true;

    primes[0] = false;

    long double sqrtt = sqrt(numt);
    for(unsigned long long l = 0; l<=sqrtt; l++) {
        if(!primes[l]) continue;
        for(unsigned long long cl = l; cl < numt; cl+= l) primes[cl] = false;
    }

    for(unsigned long long l = 0; l<numt; l++) if(primes[l]) cout << l;

    return 0;
}

Please also inform me of any bad programming practices too, please.

Answer 1

1) You didn't assign numt a value, yet you use it to initialize the vector.

2) You don't check if l is out of bounds for primes[l] here (what if numt equals 1?):

for(unsigned long long l = 0; l<=sqrtt; l++) {
    if(!primes[l]) continue;

Answer 2

Very bad practice: Compiling your code without all reasonable compiler warnings turned on, or ignoring compiler warnings.

The variable numt has no defined value. vector primes (numt) will therefore fail.

Answer 3

numt is not initialized, which means it can have any value, probably a big one. Which is problematic if you want to create a vector of that length, which causes the exception to be thrown.

Answer 4

You're not writing to numt so you probably get some ridiculously huge garbage value. When the line vector<bool> primes(numt) is evaluated, it tries to create a ridiculously huge dynamic array using the garbage value, which your OS fails to do because of excessive size.